Make GrPipeline hold a GrRenderTargetProxy (instead of a GrRenderTarget)

In a future world where GrSurface no longer has an origin it will be
useful for the GrPipeline to be holding the GrRenderTargetProxy (which will
still have an origin).

Change-Id: I743a8cc07b6b92f8116227fb77b7c37da43cde8a
Reviewed-on: https://skia-review.googlesource.com/26804
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/tests/GrMeshTest.cpp b/tests/GrMeshTest.cpp
index 0e67acf..922a996 100644
--- a/tests/GrMeshTest.cpp
+++ b/tests/GrMeshTest.cpp
@@ -367,8 +367,8 @@
 }
 
 void DrawMeshHelper::drawMesh(const GrMesh& mesh) {
-    GrRenderTarget* rt = fState->drawOpArgs().fRenderTarget;
-    GrPipeline pipeline(rt, GrPipeline::ScissorState::kDisabled, SkBlendMode::kSrc);
+    GrRenderTargetProxy* proxy = fState->drawOpArgs().fProxy;
+    GrPipeline pipeline(proxy, GrPipeline::ScissorState::kDisabled, SkBlendMode::kSrc);
     GrMeshTestProcessor mtp(mesh.isInstanced(), mesh.hasVertexData());
     fState->commandBuffer()->draw(pipeline, mtp, &mesh, nullptr, 1,
                                   SkRect::MakeIWH(kImageWidth, kImageHeight));