Switch GrTextureStripAtlas over to GrTextureProxies

This is split out of: https://skia-review.googlesource.com/c/8823/ (Remove GrFragmentProcessor-derived class' GrTexture-based ctors)

Change-Id: I9f602985b6010fc58b595e2be6d4e67e50179747
Reviewed-on: https://skia-review.googlesource.com/8881
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
13 files changed