Don't purge resources for trivial GrContext flushes

src/gpu/GrDrawingManager.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 GrDrawingManager_DEFINED
 #define GrDrawingManager_DEFINED
 
@@ skipping 50 matching lines @@
         , fAtlasTextContext(nullptr)
         , fPathRendererChain(nullptr)
         , fSoftwarePathRenderer(nullptr)
         , fFlushState(context->getGpu(), context->resourceProvider())
         , fFlushing(false) {
     }
 
     void abandon();
     void cleanup();
     void reset();
-    void flush();
+    /** Returns true if there was anything to flush and false otherwise */
+    bool flush();
 
     friend class GrContext;  // for access to: ctor, abandon, reset & flush
 
     static const int kNumPixelGeometries = 5; // The different pixel geometries
     static const int kNumDFTOptions = 2;      // DFT or no DFT
 
     GrContext*                        fContext;
     GrDrawTarget::Options             fOptionsForDrawTargets;
 
     // In debug builds we guard against improper thread handling
     GrSingleOwner*                    fSingleOwner;
 
     bool                              fAbandoned;
     SkTDArray<GrDrawTarget*>          fDrawTargets;
 
     SkAutoTDelete<GrAtlasTextContext> fAtlasTextContext;
 
     GrPathRendererChain*              fPathRendererChain;
     GrSoftwarePathRenderer*           fSoftwarePathRenderer;
 
     GrBatchFlushState                 fFlushState;
     bool                              fFlushing;
 };
 
 #endif