Add support for ANGLE_platform_angle. BUG=angle:490 Change-Id: If3c897a9ae3d27b96e4b9bf9475a9ac23a1090ba Reviewed-on: https://chromium-review.googlesource.com/185396 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>