Remove duplicate IsStringASCII call.

Most methods need the check because they then call ConvertUnicode
but this particular method calls another UTF16ToUTF8 method which
will also perform the check and appropriate conversion.

BUG=

Review-Url: https://codereview.chromium.org/2560653002
Cr-Commit-Position: refs/heads/master@{#436809}

base/strings/utf_string_conversions.cc

@@ -180,10 +180,6 @@ bool UTF16ToUTF8(const char16* src, size_t src_len, std::string* output) {
 }
 
 std::string UTF16ToUTF8(StringPiece16 utf16) {
-  if (IsStringASCII(utf16)) {
-    return std::string(utf16.begin(), utf16.end());
-  }
-
   std::string ret;
   // Ignore the success flag of this call, it will do the best it can for
   // invalid input, which is what we want here.