Make getUniqueKey virtual on GrSurfaceProxy

This lets GrResourceAllocator stop worrying about whether
proxies are texture-proxies or not.

Bug: skia:10877
Change-Id: Iccc4505bdc9d5c74f6ab7f93c26df5f913fff339
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/385497
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Adlai Holler <adlai@google.com>
4 files changed