Use ImageIndex in getRenderTargetSerial. Also move getRenderTargetSerial to the TextureStorageInterface base class, since it shares a common interface. BUG=angle:741 Change-Id: I1bc1cfac6426e241ac91d373884a7dd8a1c5b188 Reviewed-on: https://chromium-review.googlesource.com/218313 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>