Move Clear logic into RendererD3D. BUG=angle:789 Change-Id: I56d4398e4c49fe980f649327b105cdba9d944a00 Reviewed-on: https://chromium-review.googlesource.com/226062 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Brandon Jones <bajones@chromium.org>