Using the newly introduced image index struct, we can simplify
our code somewhat in TextureStorage.

BUG=angle:729

Change-Id: Iff274fd1df7a0dc36dd34e0cb31ad42831b4f5d5
Reviewed-on: https://chromium-review.googlesource.com/219836Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>