Moving this out of the Framebuffer class allows us to forward-
declare it.

BUG=angleproject:1363

Change-Id: I91971c37a92151df508cdf7f0eb8c3e93506d112
Reviewed-on: https://chromium-review.googlesource.com/340741Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>