Specify "/utf-8" with Visual C++.

This sets the source and executable character set to utf-8. This avoids
issues with local code pages and avoids adding an unwanted BOM.

Change-Id: If854c0001c2363f3262d20e28dce30c1e733536a
Reviewed-on: https://skia-review.googlesource.com/6547
Commit-Queue: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>

gn/BUILD.gn

@@ -67,6 +67,7 @@ config("default") {
       "/FS",  # Preserve previous PDB behavior.
       "/bigobj",  # Some of our files are bigger than the regular limits.
       "/WX",  # Treat warnings as errors.
+      "/utf-8",  # Set Source and Executable character sets to UTF-8.
     ]
     defines += [
       "_CRT_SECURE_NO_WARNINGS",  # Disables warnings about sscanf().