Rename GrDrawContext to GrRenderTargetContext

This is in preparation for GrTextureContext and GrSurfaceContext

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4030

Change-Id: Ie58c93052e68f3f1f5fe8d15d63760de274a6fbd
Reviewed-on: https://skia-review.googlesource.com/4030
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
diff --git a/gm/gaussianedge.cpp b/gm/gaussianedge.cpp
index bed42af..7a45118 100644
--- a/gm/gaussianedge.cpp
+++ b/gm/gaussianedge.cpp
@@ -186,8 +186,9 @@
     }
 
     void onDraw(SkCanvas* canvas) override {
-        GrDrawContext* drawContext = canvas->internal_private_accessTopLayerDrawContext();
-        if (!drawContext) {
+        GrRenderTargetContext* renderTargetContext =
+            canvas->internal_private_accessTopLayerRenderTargetContext();
+        if (!renderTargetContext) {
             skiagm::GM::DrawGpuOnlyMessage(canvas);
             return;
         }