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/src/core/SkCanvas.cpp b/src/core/SkCanvas.cpp
index 7c9f5c6..4e74a77 100644
--- a/src/core/SkCanvas.cpp
+++ b/src/core/SkCanvas.cpp
@@ -1813,9 +1813,9 @@
     return fMCRec->fRasterClip.forceGetBW();
 }
 
-GrDrawContext* SkCanvas::internal_private_accessTopLayerDrawContext() {
+GrRenderTargetContext* SkCanvas::internal_private_accessTopLayerRenderTargetContext() {
     SkBaseDevice* dev = this->getTopDevice();
-    return dev ? dev->accessDrawContext() : nullptr;
+    return dev ? dev->accessRenderTargetContext() : nullptr;
 }
 
 GrContext* SkCanvas::getGrContext() {