In all places where we called this method, we treated a NULL return
value as an internal error. This implies to me that we don't need the
two getRenderTarget and getDepthStencil methods, since we aren't using
them to check if a surface is depth or stencil.

BUG=angle:732

Change-Id: I4d1dc148abf3383b1b101bbff4f4d22de27ad03e
Reviewed-on: https://chromium-review.googlesource.com/213852Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>