Add GrRenderTargetContext instantiate & asTextureProxy

This CL also centralizes the instantiation code in GrSurfaceProxy and adds a test.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4494

Change-Id: I0081d9a216dc0af293179f23bcb88acf6a822324
Reviewed-on: https://skia-review.googlesource.com/4494
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
11 files changed