base/allocator: Clean up base/BUILD.gn about use_allocator_shim

allocator/allocator_shim_override_cpp_symbols.h and
allocator/allocator_shim_override_libc_symbols.h are not used on
all platforms.  The old BUILD.gn looked confusing as if these
two files are used regardless of platforms.

Moves the two files to platform-specific sections so that it's
clear that the files are platform-dependent.

Change-Id: I9da23cd7377d01b0a9c968224ddea88dd4c67211
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461186
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816162}

base/BUILD.gn

@@ -1290,19 +1290,23 @@ component("base") {
       "allocator/allocator_shim.cc",
       "allocator/allocator_shim.h",
       "allocator/allocator_shim_internals.h",
-      "allocator/allocator_shim_override_cpp_symbols.h",
-      "allocator/allocator_shim_override_libc_symbols.h",
     ]
     if (is_android) {
-      sources +=
-          [ "allocator/allocator_shim_override_linker_wrapped_symbols.h" ]
+      sources += [
+        "allocator/allocator_shim_override_cpp_symbols.h",
+        "allocator/allocator_shim_override_linker_wrapped_symbols.h",
+      ]
       all_dependent_configs += [ "//base/allocator:wrap_malloc_symbols" ]
     }
     if (is_apple) {
       sources += [ "allocator/allocator_shim_override_mac_symbols.h" ]
     }
     if (is_chromeos || is_linux) {
-      sources += [ "allocator/allocator_shim_override_glibc_weak_symbols.h" ]
+      sources += [
+        "allocator/allocator_shim_override_cpp_symbols.h",
+        "allocator/allocator_shim_override_glibc_weak_symbols.h",
+        "allocator/allocator_shim_override_libc_symbols.h",
+      ]
     }
     if (is_win) {
       sources += [