Also split the ANGLE modifications to the header into a separate
include.

BUG=angleproject:1897

Change-Id: I06cdcc26bab6bfa4b5bcb488c46558583cab5a83
Reviewed-on: https://chromium-review.googlesource.com/560020Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>