Make some FramebufferAttachment methods const.

BUG=angle:841

Change-Id: I4670e417c1976ebd2198ea4c8927a00fff7bb15c
Reviewed-on: https://chromium-review.googlesource.com/232380
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>