Also refactors some memory index searching code that was duplicated.
This will lead the way to having more code reuse for our Renderbuffers
implementation in Vulkan, and for other types of Texture.

Bug: angleproject:2347
Change-Id: I49cbd77328c01f945d66f92e6ec4ba7c552abeff
Reviewed-on: https://chromium-review.googlesource.com/904684
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>