Add native caching of uniquely keyed GrTextureProxies (take 2)

TBR=bsalomon@google.com

Change-Id: I590dcdc85fb60706c7eb06277694791dc04c9141
Reviewed-on: https://skia-review.googlesource.com/49543
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
18 files changed