This would drastically slow down some framebuffer updates.

BUG=angleproject:1155

Change-Id: Ibff428c49bd5127a101bce46cc1df355f4542ccf
Reviewed-on: https://chromium-review.googlesource.com/649986
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>