Don't purge resources for trivial GrContext flushes

src/gpu/GrDrawTarget.h

Index: src/gpu/GrDrawTarget.h
diff --git a/src/gpu/GrDrawTarget.h b/src/gpu/GrDrawTarget.h
index 682dd2de3300fdf718db2acb26a780c810259977..dfc1489d355099d96b317ecfc32e5aeabe3cf0ad 100644
--- a/src/gpu/GrDrawTarget.h
+++ b/src/gpu/GrDrawTarget.h
@@ -96,10 +96,11 @@ public:
     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.