Don't purge resources for trivial GrContext flushes

src/gpu/GrDrawTarget.h

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrDrawTarget_DEFINED
 #define GrDrawTarget_DEFINED
 
@@ skipping 78 matching lines @@
      * Dump out the drawTarget dependency DAG
      */
     SkDEBUGCODE(void dump() const;)
 
     /**
      * Empties the draw buffer of any queued up draws.
      */
     void reset();
 
     /**
-     * Together these two functions flush all queued up draws to the Gpu.
+     * Together these two functions flush all queued up draws to GrCommandBuffer. The return value
+     * of drawBatches() indicates whether any commands were actually issued to the GPU.
      */
     void prepareBatches(GrBatchFlushState* flushState);
-    void drawBatches(GrBatchFlushState* flushState);
+    bool drawBatches(GrBatchFlushState* flushState);
 
     /**
      * Gets the capabilities of the draw target.
      */
     const GrCaps* caps() const { return fGpu->caps(); }
 
     void drawBatch(const GrPipelineBuilder&, GrDrawContext*, const GrClip&, GrDrawBatch*);
 
     void addBatch(sk_sp<GrBatch>);
 
@@ skipping 126 matching lines @@
     bool                                            fDrawBatchBounds;
     int                                             fMaxBatchLookback;
     int                                             fMaxBatchLookahead;
 
     SkAutoTDelete<gr_instanced::InstancedRendering> fInstancedRendering;
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif