Also change the image association methods in TexStorage to use
ImageIndex, for consistency and compatibility.

BUG=angle:729

Change-Id: I05afa803735c9b93ae7d623fee5e6899c9ccec28
Reviewed-on: https://chromium-review.googlesource.com/221722Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>