cc: Don't pass simple well-defined classes by reference.

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 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/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 
 #include "base/basictypes.h"
 #include "base/debug/trace_event.h"
@@ skipping 703 matching lines @@
   if (!time_source_client_adapter_) {
     time_source_client_adapter_ = LayerTreeHostImplTimeSourceAdapter::Create(
         this,
         DelayBasedTimeSource::Create(LowFrequencyAnimationInterval(),
                                      proxy_->CurrentThread()));
   }
 
   time_source_client_adapter_->SetActive(enabled);
 }
 
-void LayerTreeHostImpl::SetViewportDamage(const gfx::Rect& damage_rect) {
+void LayerTreeHostImpl::SetViewportDamage(gfx::Rect damage_rect) {
   viewport_damage_rect_.Union(damage_rect);
 }
 
 static inline RenderPass* FindRenderPassById(
     RenderPass::Id render_pass_id,
     const LayerTreeHostImpl::FrameData& frame) {
   RenderPassIdHashMap::const_iterator it =
       frame.render_passes_by_id.find(render_pass_id);
   return it != frame.render_passes_by_id.end() ? it->second : NULL;
 }
@@ skipping 1386 matching lines @@
 }
 
 void LayerTreeHostImpl::SetDebugState(const LayerTreeDebugState& debug_state) {
   if (debug_state_.continuous_painting != debug_state.continuous_painting)
     paint_time_counter_->ClearHistory();
 
   debug_state_ = debug_state;
 }
 
 }  // namespace cc