Back SkSpecialImage_Gpu with a GrTextureProxy

This is split out of https://codereview.chromium.org/2215323003/ (Start using RenderTargetProxy (omnibus))

The addition of the gpuMemorySize methods is for the SkSpecialImage cache.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4227

Change-Id: Ia9b9d42fb2a0caf61bbfa3ebcc84308c56f541fc
Reviewed-on: https://skia-review.googlesource.com/4227
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
10 files changed