The format is still passed as BGRA to the driver so that the data can be
properly unpacked.

BUG=angleproject:884

Change-Id: I767626c818ce1a3c5a4739f07aa623bf8a9ae377
Reviewed-on: https://chromium-review.googlesource.com/273162Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>