Don't make unnecessary copy of ChildToParentMap

cc/surfaces/surface_aggregator.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_aggregator.h"
 
 #include <map>
 
 #include "base/bind.h"
 #include "base/containers/hash_tables.h"
@@ skipping 210 matching lines @@
     for (auto& request : copy_requests) {
       request.second->SendEmptyResult();
       delete request.second;
     }
     return;
   }
 
   SurfaceSet::iterator it = referenced_surfaces_.insert(surface_id).first;
   // TODO(vmpstr): provider check is a hack for unittests that don't set up a
   // resource provider.
+  ResourceProvider::ResourceIdMap empty_map;
   const ResourceProvider::ResourceIdMap& child_to_parent_map =
       provider_ ? provider_->GetChildToParentMap(ChildIdForSurface(surface))
-                : ResourceProvider::ResourceIdMap();
+                : empty_map;
   bool merge_pass =
       surface_quad->shared_quad_state->opacity == 1.f && copy_requests.empty();
 
   gfx::Rect surface_damage = DamageRectForSurface(
       surface, *render_pass_list.back(), surface_quad->visible_rect);
   const RenderPassList& referenced_passes = render_pass_list;
   size_t passes_to_copy =
       merge_pass ? referenced_passes.size() - 1 : referenced_passes.size();
   for (size_t j = 0; j < passes_to_copy; ++j) {
     const RenderPass& source = *referenced_passes[j];
@@ skipping 188 matching lines @@
   surface->TakeCopyOutputRequests(&copy_requests);
 
   const RenderPassList& source_pass_list = frame_data->render_pass_list;
   bool invalid_frame = ValidateResources(surface, frame_data);
   DCHECK(!invalid_frame);
   if (invalid_frame)
     return;
 
   // TODO(vmpstr): provider check is a hack for unittests that don't set up a
   // resource provider.
+  ResourceProvider::ResourceIdMap empty_map;
   const ResourceProvider::ResourceIdMap& child_to_parent_map =
       provider_ ? provider_->GetChildToParentMap(ChildIdForSurface(surface))
-                : ResourceProvider::ResourceIdMap();
+                : empty_map;
   for (size_t i = 0; i < source_pass_list.size(); ++i) {
     const RenderPass& source = *source_pass_list[i];
 
     size_t sqs_size = source.shared_quad_state_list.size();
     size_t dq_size = source.quad_list.size();
     scoped_ptr<RenderPass> copy_pass(RenderPass::Create(sqs_size, dq_size));
 
     MoveMatchingRequests(source.id, &copy_requests, &copy_pass->copy_requests);
 
     RenderPassId remapped_pass_id =
@@ skipping 89 matching lines @@
 
 void SurfaceAggregator::SetFullDamageForSurface(SurfaceId surface_id) {
   auto it = previous_contained_surfaces_.find(surface_id);
   if (it == previous_contained_surfaces_.end())
     return;
   // Set the last drawn index as 0 to ensure full damage next time it's drawn.
   it->second = 0;
 }
 
 }  // namespace cc