Add ability to remove unique key from proxy and underlying surface.

Bug: skia:
Change-Id: I66b891ce9ca35906fdbddb36f565b35b25825112
Reviewed-on: https://skia-review.googlesource.com/51240
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
18 files changed