*re-land with compile fix*

We would spend a fair bit of time iterating over the unused textures,
setting them to null one-by-one, and updating our cache. We can
reduce this time by smarter caching, and skipping unmodified ranges.

BUG=angleproject:959

Change-Id: I4de20bc131c4a568108ad670a2ef491cfd4c50ed
Reviewed-on: https://chromium-review.googlesource.com/280916Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>