cc: Don't draw and swap if the frame will not change.

cc/trees/damage_tracker.h

 // 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.
 
 #ifndef CC_TREES_DAMAGE_TRACKER_H_
 #define CC_TREES_DAMAGE_TRACKER_H_
 
 #include <vector>
 #include "base/hash_tables.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 17 matching lines @@
 
 // Computes the region where pixels have actually changed on a
 // RenderSurfaceImpl. This region is used to scissor what is actually drawn to
 // the screen to save GPU computation and bandwidth.
 class CC_EXPORT DamageTracker {
  public:
   static scoped_ptr<DamageTracker> Create();
   ~DamageTracker();
 
   void DidDrawDamagedArea() { current_damage_rect_ = gfx::RectF(); }
-  void ForceFullDamageNextUpdate() { force_full_damage_next_update_ = true; }
+  void AddDamageNextUpdate(gfx::RectF dmg) { current_damage_rect_.Union(dmg); }
   void UpdateDamageTrackingState(
       const std::vector<LayerImpl*>& layer_list,
       int target_surface_layer_id,
       bool target_surface_property_changed_only_from_descendant,
       gfx::Rect target_surface_content_rect,
       LayerImpl* target_surface_mask_layer,
       const WebKit::WebFilterOperations& filters,
       SkImageFilter* filter);
 
   gfx::RectF current_damage_rect() { return current_damage_rect_; }
@@ skipping 17 matching lines @@
 
   // To correctly track exposed regions, two hashtables of rects are maintained.
   // The "current" map is used to compute exposed regions of the current frame,
   // while the "next" map is used to collect layer rects that are used in the
   // next frame.
   typedef base::hash_map<int, gfx::RectF> RectMap;
   scoped_ptr<RectMap> current_rect_history_;
   scoped_ptr<RectMap> next_rect_history_;
 
   gfx::RectF current_damage_rect_;
-  bool force_full_damage_next_update_;
 
   DISALLOW_COPY_AND_ASSIGN(DamageTracker);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_DAMAGE_TRACKER_H_