Don't purge resources for trivial GrContext flushes

src/gpu/GrDrawingManager.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrDrawContext.h"
 #include "GrDrawingManager.h"
 #include "GrDrawTarget.h"
@@ skipping 56 matching lines @@
     }
 }
 
 void GrDrawingManager::reset() {
     for (int i = 0; i < fDrawTargets.count(); ++i) {
         fDrawTargets[i]->reset();
     }
     fFlushState.reset();
 }
 
-void GrDrawingManager::flush() {
+bool GrDrawingManager::flush() {
     if (fFlushing || this->wasAbandoned()) {
-        return;
+        return false;
     }
     fFlushing = true;
-
+    bool flushed = false;
     SkDEBUGCODE(bool result =)
                         SkTTopoSort<GrDrawTarget, GrDrawTarget::TopoSortTraits>(&fDrawTargets);
     SkASSERT(result);
 
     for (int i = 0; i < fDrawTargets.count(); ++i) {
         fDrawTargets[i]->prepareBatches(&fFlushState);
     }
 
     // Enable this to print out verbose batching information
 #if 0
     for (int i = 0; i < fDrawTargets.count(); ++i) {
         SkDEBUGCODE(fDrawTargets[i]->dump();)
     }
 #endif
 
     // Upload all data to the GPU
     fFlushState.preIssueDraws();
 
     for (int i = 0; i < fDrawTargets.count(); ++i) {
-        fDrawTargets[i]->drawBatches(&fFlushState);
+        if (fDrawTargets[i]->drawBatches(&fFlushState)) {
+            flushed = true;
+        }
     }
 
     SkASSERT(fFlushState.nextDrawToken() == fFlushState.nextTokenToFlush());
 
     for (int i = 0; i < fDrawTargets.count(); ++i) {
         fDrawTargets[i]->reset();
 #ifdef ENABLE_MDB
         fDrawTargets[i]->unref();
 #endif
     }
 
 #ifndef ENABLE_MDB
     // When MDB is disabled we keep reusing the same drawTarget
     if (fDrawTargets.count()) {
         SkASSERT(fDrawTargets.count() == 1);
         // Clear out this flag so the topological sort's SkTTopoSort_CheckAllUnmarked check
         // won't bark
         fDrawTargets[0]->resetFlag(GrDrawTarget::kWasOutput_Flag);
     }
 #else
     fDrawTargets.reset();
 #endif
 
     fFlushState.reset();
     fFlushing = false;
+    return flushed;
 }
 
 GrDrawTarget* GrDrawingManager::newDrawTarget(GrRenderTarget* rt) {
     SkASSERT(fContext);
 
 #ifndef ENABLE_MDB
     // When MDB is disabled we always just return the single drawTarget
     if (fDrawTargets.count()) {
         SkASSERT(fDrawTargets.count() == 1);
         // In the non-MDB-world the same drawTarget gets reused for multiple render targets.
@@ skipping 75 matching lines @@
                                                         std::move(colorSpace), surfaceProps,
                                                         fContext->getAuditTrail(), fSingleOwner));
         }
     }
 
     return sk_sp<GrDrawContext>(new GrDrawContext(fContext, this, std::move(rt),
                                                   std::move(colorSpace), surfaceProps,
                                                   fContext->getAuditTrail(),
                                                   fSingleOwner));
 }