Set enable_swiftshader arg on Chrome-branded windows builds.

This matches the previous behavior with gyp (which was necessitated by
the need for a registration key). In the future we can expand this to
other platforms and to regular Chromium.

BUG=646660

Review-Url: https://codereview.chromium.org/2345783003
Cr-Commit-Position: refs/heads/master@{#419032}

ui/gl/BUILD.gn

@@ -10,7 +10,7 @@ import("//ui/ozone/ozone.gni")
 import("//testing/test.gni")
 
 declare_args() {
-  enable_swiftshader = false
+  enable_swiftshader = is_chrome_branded && is_win
 }
 
 use_egl = is_win || is_android || is_linux