This encapsulates the three values needed to index into the image
array. It will simplify the logic for querying texture images from
the base calss.

BUG=angle:732

Change-Id: I31c55b3f972fd4d96ab540ec8498ef4b9b2ba16b
Reviewed-on: https://chromium-review.googlesource.com/213856Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>