Start using GrGpuCommandBuffer in GrDrawTarget.

src/gpu/batches/GrDiscardBatch.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrDiscardBatch_DEFINED
 #define GrDiscardBatch_DEFINED
 
@@ skipping 24 matching lines @@
     }
 
 private:
     bool onCombineIfPossible(GrBatch* that, const GrCaps& caps) override {
         return fRenderTarget == that->cast<GrDiscardBatch>()->fRenderTarget;
     }
 
     void onPrepare(GrBatchFlushState*) override {}
 
     void onDraw(GrBatchFlushState* state) override {
-        state->gpu()->discard(fRenderTarget.get());
+        state->commandBuffer()->discard(fRenderTarget.get());
     }
 
     GrPendingIOResource<GrRenderTarget, kWrite_GrIOType> fRenderTarget;
 
     typedef GrBatch INHERITED;
 };
 
 #endif