This flag we used in both D3D renderers, and hence we can move it
to the base class to share a bit of common code.

BUG=angle:795

Change-Id: I8bd60796bfeb9590bdf798f9d044f8e20c7ad779
Reviewed-on: https://chromium-review.googlesource.com/228912Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>