It was overkill to always destroy the storage, especially for cube maps.
Instead, just make sure the image thinks it's dirty when robust resource
init is enabled so that it is always copied to the storage after
redefinition.

BUG=angleproject:1635

Change-Id: Ieba065d1873ca66a216bd05f4f8afa9c75349ff9
Reviewed-on: https://chromium-review.googlesource.com/526295Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>