Make GrDrawContext be GrRenderTargetProxy-backed

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

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3841

CQ_INCLUDE_TRYBOTS=master.client.skia.android:Test-Android-Clang-Nexus5-GPU-Adreno330-arm-Debug-GN_Android-Trybot

Change-Id: I1a47f19ed1ac0c249e6ccac8db74095d7f456db4
Reviewed-on: https://skia-review.googlesource.com/3841
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/tests/ProxyTest.cpp b/tests/ProxyTest.cpp
index 367dee4..9a7697c 100644
--- a/tests/ProxyTest.cpp
+++ b/tests/ProxyTest.cpp
@@ -13,6 +13,7 @@
 #include "GrGpu.h"
 #include "GrSurfaceProxy.h"
 #include "GrTextureProxy.h"
+#include "GrRenderTargetPriv.h"
 #include "GrRenderTargetProxy.h"
 
 // Check that the surface proxy's member vars are set as expected