The state layer should not be querying render targets directly,
since they are not a GL concept.

BUG=angle:732

Change-Id: I5ee96ae54cced8a32c5082ddd9f8796a975e2591
Reviewed-on: https://chromium-review.googlesource.com/213971Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>