These Renderer methods can be replaced with virtual TextureStorage
methods, now that we do not have TextureStorageInterface to
complicate matters. This reduces the number of stubs in Renderer
as well as allowing cleaner code re-use.

BUG=angle:729

Change-Id: I6d2004d4f1abdb1041420144cee1c173e5ab199e
Reviewed-on: https://chromium-review.googlesource.com/219839Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>