OSX10.8 deprecates Gestalt but doesn't expose a replacement in the
public interface so we forward declare it.

BUG=angleproject:1890

Change-Id: Ie4825c9191e81a5894dca4e394275c26a3e27853
Reviewed-on: https://chromium-review.googlesource.com/441309
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>