This paves the way for setting data on the TextureStorage directly
instead of working through the Image objects.

BUG=angle:741

Change-Id: I3be3d5f9b2e45707c1630b74ad3f4789e034c3fd
Reviewed-on: https://chromium-review.googlesource.com/218311Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>