Clean up test drawContext usage

include/core/SkDevice.h

Index: include/core/SkDevice.h
diff --git a/include/core/SkDevice.h b/include/core/SkDevice.h
index 73c35c33419f0934797ae7994ab1732b4c35558a..e06650036dc10b35e8e3528f0153f6673c617bc2 100644
--- a/include/core/SkDevice.h
+++ b/include/core/SkDevice.h
@@ -106,8 +106,12 @@ public:
     /**
      * Return the device's associated gpu render target, or NULL.
      */
-    virtual GrRenderTarget* accessRenderTarget() { return NULL; }
+    virtual GrRenderTarget* accessRenderTarget() { return nullptr; }
 
+    /**
+     * Don't call this!
+     */
+    virtual GrDrawContext* accessDrawContext() { return nullptr; }
 
     /**
      *  Return the device's origin: its offset in device coordinates from