Specify the default framebuffer formats in GL formats but still use BGRA
and ANGLEX formats for now.

BUG=angle:658

Change-Id: I7192db4ca76ab4b0b42daa43785a7ddd9528a9ca
Reviewed-on: https://chromium-review.googlesource.com/239902Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>