This allows the Framebuffer to initialize all the attachment info
internally. This in turn will allow us to clean up the way we
validate WebGL1 depth/stencil attachments.

BUG=angleproject:1708

Change-Id: I8871a9791dfce0ac806f4e1367a521610e5283ae
Reviewed-on: https://chromium-review.googlesource.com/446130Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>