adding preabandon flag to DM

BUG=skia:

Review URL: https://codereview.chromium.org/929243004
diff --git a/src/gpu/SkGpuDevice.h b/src/gpu/SkGpuDevice.h
index 8dafcfc..6b520e7 100644
--- a/src/gpu/SkGpuDevice.h
+++ b/src/gpu/SkGpuDevice.h
@@ -57,7 +57,7 @@
         return static_cast<SkGpuDevice*>(dev);
     }
 
-    GrContext* context() const { return fRenderTarget->getContext(); }
+    GrContext* context() const { return fContext; }
 
     // set all pixels to 0
     void clearAll();