Use GrTextureProvider's uniqueKey setting method rather than directly setting it

Clients will not be able to directly set the uniqueKey on GrTextureProxies. This CL sets up the choke point for the switch over to having uniqueKeys be managed by a third party (the textureProvider).

Change-Id: I5061a970faf77ea0c4a320e021ff7c3ef90a0900
Reviewed-on: https://skia-review.googlesource.com/9140
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
8 files changed