Reland the commit after fixing a bug in FramebufferAttachment.

This reverts commit 18fdcbcf.

BUG=angleproject:891

Change-Id: I07e08de52bfce8d84d070fc7bc15883009298a4d
Reviewed-on: https://chromium-review.googlesource.com/294831Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>