After testing, it seems practially impossible to trigger a
dependent state change in a Buffer that would affect XFB.
Any buffer change would have to happen between a pause or
begin/end of XFB, which would trigger a state update.

Bug: angleproject:1155
Bug: angleproject:2389
Change-Id: Ic3c1dc7cec661a826909bb5647ddabda1d6fb7fc
Reviewed-on: https://chromium-review.googlesource.com/948795Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>