Add GrTextureRenderTargetProxy and support switching between RT & Tex Proxies

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4427

Change-Id: Ie7662299953592f564bb27a4df4ea101f743403e
Reviewed-on: https://skia-review.googlesource.com/4427
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index f300677..cc72f4b 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -62,6 +62,7 @@
   "$_include/private/GrRenderTargetProxy.h",
   "$_include/private/GrSurfaceProxy.h",
   "$_include/private/GrTextureProxy.h",
+  "$_include/private/GrTextureRenderTargetProxy.h",
   "$_include/private/GrTextureStripAtlas.h",
 
   "$_src/gpu/GrAppliedClip.h",
@@ -200,6 +201,7 @@
   "$_src/gpu/GrTexturePriv.h",
   "$_src/gpu/GrTextureProvider.cpp",
   "$_src/gpu/GrTextureProxy.cpp",
+  "$_src/gpu/GrTextureRenderTargetProxy.cpp",
   "$_src/gpu/GrTextureToYUVPlanes.cpp",
   "$_src/gpu/GrTextureToYUVPlanes.h",
   "$_src/gpu/GrTextureAccess.cpp",