Reland "[base] Change string16 to std::u16string everywhere"

This is a reland of 7c12965

In addition to the initial changes it addresses the compiliation failure
in tools/ipc_fuzzer/message_dump/message_dump.cc.

Original change's description:
> [base] Change string16 to std::u16string everywhere
>
> This change modifies base::string16 to be an alias of std::u16string
> on all platforms. base::char16, blink::WebUChar and the STRING16_LITERAL
> macro are modified accordingly.
>
> Bug: 911896
> Change-Id: I15aec7b9f50288291219426857e8a1b5d93ad852
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1776026
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Commit-Queue: Daniel Cheng <dcheng@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#858728}

Bug: 911896
Change-Id: I87ab070e1f865ae4c7d0fb518855c2bb51de3958
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2727825
Owners-Override: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#859028}

base/strings/string16.h

@@ -18,32 +18,21 @@
 
 #include <string>
 
-#include "build/build_config.h"
-
-#if defined(WCHAR_T_IS_UTF16)
-
 // Define a macro for wrapping construction of char16 arrays and string16s from
 // a literal string. This indirection allows for an easier migration of
 // base::char16 to char16_t on platforms where WCHAR_T_IS_UTF16, as only a one
 // character change to the macro will be necessary.
 // TODO(https://crbug.com/911896): Remove this macro once base::char16 is
 // char16_t on all platforms.
-#define STRING16_LITERAL(x) L##x
-
-namespace base {
-using char16 = wchar_t;
-using string16 = std::wstring;
-}  // namespace base
-
-#else
-
 #define STRING16_LITERAL(x) u##x
 
 namespace base {
 using char16 = char16_t;
 using string16 = std::u16string;
 }  // namespace base
 
-#endif  // WCHAR_T_IS_UTF16
+#ifndef BASE_STRING16_IS_STD_U16STRING
+#define BASE_STRING16_IS_STD_U16STRING
+#endif  // BASE_STRING16_IS_STD_U16STRING
 
 #endif  // BASE_STRINGS_STRING16_H_

base/strings/string_piece.cc

@@ -39,6 +39,7 @@ inline void BuildLookupTable(StringPiece characters_wanted, bool* table) {
 #if !defined(COMPILER_MSVC)
 template class BasicStringPiece<std::string>;
 template class BasicStringPiece<string16>;
+template class BasicStringPiece<std::wstring>;
 #endif
 
 std::ostream& operator<<(std::ostream& o, StringPiece piece) {
@@ -50,11 +51,9 @@ std::ostream& operator<<(std::ostream& o, StringPiece16 piece) {
   return o << UTF16ToUTF8(piece);
 }
 
-#if !defined(WCHAR_T_IS_UTF16)
 std::ostream& operator<<(std::ostream& o, WStringPiece piece) {
   return o << WideToUTF8(piece);
 }
-#endif
 
 namespace internal {
 
@@ -284,7 +283,6 @@ size_t find_last_not_of(StringPiece16 self, StringPiece16 s, size_t pos) {
   return find_last_not_ofT(self, s, pos);
 }
 
-#if !defined(WCHAR_T_IS_UTF16)
 size_t find(WStringPiece self, WStringPiece s, size_t pos) {
   return findT(self, s, pos);
 }
@@ -308,7 +306,5 @@ size_t find_last_of(WStringPiece self, WStringPiece s, size_t pos) {
 size_t find_last_not_of(WStringPiece self, WStringPiece s, size_t pos) {
   return find_last_not_ofT(self, s, pos);
 }
-#endif
-
 }  // namespace internal
 }  // namespace base

base/strings/string_piece.h

@@ -79,8 +79,6 @@ BASE_EXPORT size_t find_last_not_of(StringPiece16 self,
                                     StringPiece16 s,
                                     size_t pos);
 
-// Overloads for WStringPiece in case it is not the same type as StringPiece16.
-#if !defined(WCHAR_T_IS_UTF16)
 BASE_EXPORT size_t find(WStringPiece self, WStringPiece s, size_t pos);
 BASE_EXPORT size_t rfind(WStringPiece self, WStringPiece s, size_t pos);
 BASE_EXPORT size_t find_first_of(WStringPiece self, WStringPiece s, size_t pos);
@@ -91,7 +89,6 @@ BASE_EXPORT size_t find_last_of(WStringPiece self, WStringPiece s, size_t pos);
 BASE_EXPORT size_t find_last_not_of(WStringPiece self,
                                     WStringPiece s,
                                     size_t pos);
-#endif
 
 }  // namespace internal
 
@@ -565,10 +562,7 @@ constexpr bool operator>=(std::common_type_t<BasicStringPiece<StringT>> lhs,
 
 BASE_EXPORT std::ostream& operator<<(std::ostream& o, StringPiece piece);
 BASE_EXPORT std::ostream& operator<<(std::ostream& o, StringPiece16 piece);
-
-#if !defined(WCHAR_T_IS_UTF16)
 BASE_EXPORT std::ostream& operator<<(std::ostream& o, WStringPiece piece);
-#endif
 
 // Hashing ---------------------------------------------------------------------
 

third_party/blink/public/platform/web_common.h

@@ -86,11 +86,7 @@ namespace blink {
 typedef int32_t WebUChar32;
 
 // UTF-16 character type
-#if defined(WIN32)
-typedef wchar_t WebUChar;
-#else
 typedef char16_t WebUChar;
-#endif
 
 // Latin-1 character type
 typedef unsigned char WebLChar;

tools/ipc_fuzzer/message_dump/message_dump.cc

@@ -11,7 +11,7 @@
 #include "tools/ipc_fuzzer/message_lib/message_file.h"
 
 #if defined(OS_WIN)
-#define PidToStringType base::NumberToString16
+#define PidToStringType base::NumberToWString
 #define MESSAGE_DUMP_EXPORT __declspec(dllexport)
 #else
 #define PidToStringType base::NumberToString