Use null rather than empty frame to lock Surface resources

cc/surfaces/surface.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/surfaces/surface.h"
 
 #include <algorithm>
 
 #include "cc/output/compositor_frame.h"
 #include "cc/output/copy_output_request.h"
@@ skipping 23 matching lines @@
     factory_->UnrefResources(current_resources);
   }
   if (!draw_callback_.is_null())
     draw_callback_.Run(SurfaceDrawStatus::DRAW_SKIPPED);
 }
 
 void Surface::QueueFrame(scoped_ptr<CompositorFrame> frame,
                          const DrawCallback& callback) {
   DCHECK(factory_);
   ClearCopyRequests();
-  TakeLatencyInfo(&frame->metadata.latency_info);
+
+  if (frame) {
+    TakeLatencyInfo(&frame->metadata.latency_info);
+  }
+
   scoped_ptr<CompositorFrame> previous_frame = current_frame_.Pass();
   current_frame_ = frame.Pass();
-  factory_->ReceiveFromChild(
-      current_frame_->delegated_frame_data->resource_list);
+
+  if (current_frame_) {
+    factory_->ReceiveFromChild(
+        current_frame_->delegated_frame_data->resource_list);
+  }
+
   // Empty frames shouldn't be drawn and shouldn't contribute damage, so don't
   // increment frame index for them.
-  if (!current_frame_ ||
+  if (current_frame_ &&
       !current_frame_->delegated_frame_data->render_pass_list.empty())
     ++frame_index_;
 
   if (previous_frame) {
     ReturnedResourceArray previous_resources;
     TransferableResource::ReturnResources(
         previous_frame->delegated_frame_data->resource_list,
         &previous_resources);
     factory_->UnrefResources(previous_resources);
   }
   if (!draw_callback_.is_null())
     draw_callback_.Run(SurfaceDrawStatus::DRAW_SKIPPED);
   draw_callback_ = callback;
-  factory_->manager()->DidSatisfySequences(
-      SurfaceIdAllocator::NamespaceForId(surface_id_),
-      &current_frame_->metadata.satisfies_sequences);
+
+  if (current_frame_) {
+    factory_->manager()->DidSatisfySequences(
+        SurfaceIdAllocator::NamespaceForId(surface_id_),
+        &current_frame_->metadata.satisfies_sequences);
+  }
 }
 
 void Surface::RequestCopyOfOutput(scoped_ptr<CopyOutputRequest> copy_request) {
   if (current_frame_ &&
       !current_frame_->delegated_frame_data->render_pass_list.empty())
     current_frame_->delegated_frame_data->render_pass_list.back()
         ->copy_requests.push_back(copy_request.Pass());
   else
     copy_request->SendEmptyResult();
 }
@@ skipping 57 matching lines @@
   if (current_frame_) {
     for (const auto& render_pass :
          current_frame_->delegated_frame_data->render_pass_list) {
       for (const auto& copy_request : render_pass->copy_requests)
         copy_request->SendEmptyResult();
     }
   }
 }
 
 }  // namespace cc