WebKit needs these preprocessor directives because they can't
selectively compile files per platform.

Bug: angleproject:4613
Change-Id: Ie549cd734c74b50666f3d7e142b885bc78077fc2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2255701
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>