Refactor Framebuffer::invalidateSub. Use our new helper methods to simplify this method. BUG=angle:732 Change-Id: I56b837678d5efbc60de6de4bdb01447ca811e71d Reviewed-on: https://chromium-review.googlesource.com/213855 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org>