This moves the bit mask of attached color buffer to front end. No
function change is expected.

Bug: b/167301719
Change-Id: I4eec3a419560b91875260b812a036ceaaeb6b5bb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2410790Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>