Revert "win: /utf-8 for all code"

This reverts commit a53f3c3734ad9ee4322f6a9a60eede7f6b04530f.

Original CL: https://codereview.chromium.org/2543743002

Reason for revert: Compilation issue on master.tryserver.chromium.win:win_optional_gpu_tests_rel

Failing [1] task on the bot:

FAILED: obj/third_party/angle/src/tests/angle_deqp_libgles3/es3pBufferDataUploadTests.obj
ninja -t msvc -e environment.x86 -- E:\b\c\cipd\goma/gomacc.exe "E:\b\depot_tools\win_toolchain\vs_files\d5dc33b15d1b2c086f2f6632e2fd15882f80dbd3\VC\bin\amd64_x86/cl.exe" /nologo /showIncludes /FC @obj/third_party/angle/src/tests/angle_deqp_libgles3/es3pBufferDataUploadTests.obj.rsp /c ../../third_party/deqp/src/modules/gles3/performance/es3pBufferDataUploadTests.cpp /Foobj/third_party/angle/src/tests/angle_deqp_libgles3/es3pBufferDataUploadTests.obj /Fd"obj/third_party/angle/src/tests/angle_deqp_libgles3_cc.pdb"
e:\b\c\b\win\src\third_party\deqp\src\modules\gles3\performance\es3pbufferdatauploadtests.cpp(5280): error C2220: warning treated as error - no 'object' file generated
e:\b\c\b\win\src\third_party\deqp\src\modules\gles3\performance\es3pbufferdatauploadtests.cpp(5280): warning C4828: The file contains a character starting at offset 0x3714f that is illegal in the current source character set (codepage 65001).
e:\b\c\b\win\src\third_party\deqp\src\modules\gles3\performance\es3pbufferdatauploadtests.cpp(5280): note: to simplify migration, consider the temporary use of /Wv:18 flag with the version of the compiler with which you used to build without warnings

[1]
https://build.chromium.org/p/tryserver.chromium.win/builders/win_optional_gpu_tests_rel/builds/5729/steps/compile%20%28without%20patch%29/logs/stdio

BUG=454858, 637203, 671021

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.win:win_optional_gpu_tests_rel
NOTRY=true
NOTREECHECKS=true
TBR=scottmg@chromium.org, dpranke@chromium.org

Review-Url: https://codereview.chromium.org/2550973002
Cr-Original-Commit-Position: refs/heads/master@{#436195}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 373af458aa9ab8f6f8650da115209c74d14f5dea

config/compiler/BUILD.gn

@@ -842,11 +842,6 @@ config("default_warnings") {
       cflags += [ "/WX" ]
     }
 
-    cflags += [
-      # Assume UTF-8 by default to avoid code page dependencies.
-      "/utf-8",
-    ]
-
     cflags += [
       # Warnings permanently disabled:
 
@@ -1078,6 +1073,9 @@ config("default_warnings") {
 config("chromium_code") {
   if (is_win) {
     cflags = [ "/W4" ]  # Warning level 4.
+
+    # Assume UTF-8 by default to avoid code page dependencies.
+    cflags += [ "/utf-8" ]
   } else {
     cflags = [ "-Wall" ]
     if (treat_warnings_as_errors) {