This allows Framebuffer syncState methods to handle dirty bits
themselves. This will be useful for the implementation of masked color
clear.

Bug: angleproject:2455
Change-Id: I65ad9c61e89e317c820f6be5550edd8185b52afc
Reviewed-on: https://chromium-review.googlesource.com/1032856
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>