Also adds a few assert in SetData and various places to ensure that
images are not dirty before modifying the storage.

BUG=angle:873

Change-Id: I7cc922b33da0d79a1b5aefe216e9ccfaaf86a306
Reviewed-on: https://chromium-review.googlesource.com/241880Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Gregoire Payen de La Garanderie <Gregory.Payen@imgtec.com>