Tighten up GrSurfaceProxy typing

This may reduce the number of "why not GrTextureProxy" issues

Change-Id: I9e0e5042f5801ba9a933b697a380cb0cb54b4522
Reviewed-on: https://skia-review.googlesource.com/8510
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
9 files changed