Was lost in https://chromium-review.googlesource.com/434188

Also, don't reset pre-set Platform to Default,
otherwise Chrome's platform gets reset.

BUG=angleproject:1660, angleproject:1892

Change-Id: I052c86c513c8d89d2420a4724a8bd0dc7446c7c2
Reviewed-on: https://chromium-review.googlesource.com/444928
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>