Don't purge resources for trivial GrContext flushes
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2298003003

Review-Url: https://codereview.chromium.org/2298003003
diff --git a/src/gpu/GrDrawTarget.h b/src/gpu/GrDrawTarget.h
index 682dd2d..dfc1489 100644
--- a/src/gpu/GrDrawTarget.h
+++ b/src/gpu/GrDrawTarget.h
@@ -96,10 +96,11 @@
     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.