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/tests.gni b/gn/tests.gni
index 6de39b0..135c3ce 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -155,6 +155,7 @@
   "$_tests/PointTest.cpp",
   "$_tests/PremulAlphaRoundTripTest.cpp",
   "$_tests/PrimitiveProcessorTest.cpp",
+  "$_tests/ProxyConversionTest.cpp",
   "$_tests/ProxyTest.cpp",
   "$_tests/QuickRejectTest.cpp",
   "$_tests/RandomTest.cpp",