cc: Don't do full tree sync unless needed with impl painting.

cc/layer_tree_impl.h

 // Copyright 2012 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_LAYER_TREE_IMPL_H_
 #define CC_LAYER_TREE_IMPL_H_
 
 #include "base/hash_tables.h"
 #include "cc/layer_impl.h"
 
@@ skipping 127 matching lines @@
 
   // Updates draw properties and render surface layer list
   void UpdateDrawProperties(UpdateDrawPropertiesReason reason);
   void set_needs_update_draw_properties() {
     needs_update_draw_properties_ = true;
   }
   bool needs_update_draw_properties() const {
     return needs_update_draw_properties_;
   }
 
+  void set_needs_full_tree_sync(bool needs) { needs_full_tree_sync_ = needs; }
+  bool needs_full_tree_sync() const { return needs_full_tree_sync_; }
+
   void ClearRenderSurfaces();
 
   bool AreVisibleResourcesReady() const;
 
   const LayerList& RenderSurfaceLayerList() const;
 
   // These return the size of the root scrollable area and the size of
   // the user-visible scrolling viewport, in CSS layout coordinates.
   gfx::Size ScrollableSize() const;
   gfx::SizeF ScrollableViewportSize() const;
@@ skipping 41 matching lines @@
   // Persisted state for non-impl-side-painting.
   int scrolling_layer_id_from_previous_tree_;
 
   // List of visible layers for the most recently prepared frame. Used for
   // rendering and input event hit testing.
   LayerList render_surface_layer_list_;
 
   bool contents_textures_purged_;
   bool needs_update_draw_properties_;
 
+  // In impl-side painting mode, this is true when the tree may contain
+  // structural differences relative to the active tree.
+  bool needs_full_tree_sync_;
+
   DISALLOW_COPY_AND_ASSIGN(LayerTreeImpl);
 };
 
 }
 
 #endif  // CC_LAYER_TREE_IMPL_H_