Don't purge resources for trivial GrContext flushes

src/gpu/GrDrawTarget.cpp

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrDrawTarget.h"
 
 #include "GrAppliedClip.h"
@@ skipping 203 matching lines @@
         if (fRecordedBatches[i].fBatch) {
             fRecordedBatches[i].fBatch->prepare(flushState);
         }
     }
 
     if (fInstancedRendering) {
         fInstancedRendering->beginFlush(flushState->resourceProvider());
     }
 }
 
-void GrDrawTarget::drawBatches(GrBatchFlushState* flushState) {
+bool GrDrawTarget::drawBatches(GrBatchFlushState* flushState) {
     if (0 == fRecordedBatches.count()) {
-        return;
+        return false;
     }
     // Draw all the generated geometry.
     SkRandom random;
     GrRenderTarget* currentRT = nullptr;
     SkAutoTDelete<GrGpuCommandBuffer> commandBuffer;
     SkRect bounds = SkRect::MakeEmpty();
     for (int i = 0; i < fRecordedBatches.count(); ++i) {
         if (!fRecordedBatches[i].fBatch) {
             continue;
         }
@@ skipping 42 matching lines @@
                              SkIntToScalar(currentRT->width()),
                              SkIntToScalar(currentRT->height()))) {
             SkIRect iBounds;
             bounds.roundOut(&iBounds);
             commandBuffer->submit(iBounds);
         }
         flushState->setCommandBuffer(nullptr);
     }
 
     fGpu->finishDrawTarget();
+    return true;
 }
 
 void GrDrawTarget::reset() {
     fLastFullClearBatch = nullptr;
     fRecordedBatches.reset();
     if (fInstancedRendering) {
         fInstancedRendering->endFlush();
     }
 }
 
@@ skipping 317 matching lines @@
     }
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
 void GrDrawTarget::clearStencilClip(const SkIRect& rect, bool insideClip, GrRenderTarget* rt) {
     GrBatch* batch = new GrClearStencilClipBatch(rect, insideClip, rt);
     this->recordBatch(batch, batch->bounds());
     batch->unref();
 }