Enable /utf-8 on Windows

Applies only to chromium_code for now. There's some characters in
third_party that cl.exe claims aren't representable in code page 65001
(which is its way of saying UTF-8 without a BOM).

R=brucedawson@chromium.org
BUG=454858,637203

Review-Url: https://codereview.chromium.org/2488853002
Cr-Original-Commit-Position: refs/heads/master@{#434212}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 63384a79de37f89a88cfc02ff61eb649e4abe535

config/compiler/BUILD.gn

@@ -1057,6 +1057,9 @@ 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) {