This method uses rx::RenderTarget, which is D3D-only.

BUG=angle:773

Change-Id: I2cb523fa98bda96661260d37a8c69c2ed8e7aa84
Reviewed-on: https://chromium-review.googlesource.com/233441Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>