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

cc/layer_tree_impl.h

Index: cc/layer_tree_impl.h
diff --git a/cc/layer_tree_impl.h b/cc/layer_tree_impl.h
index 139b5f98c898d3b6fedf72c513cdf2c4a9b7a7a1..e9b14c72ebbb075125df39ad91f712a4023a8824 100644
--- a/cc/layer_tree_impl.h
+++ b/cc/layer_tree_impl.h
@@ -145,6 +145,9 @@ class CC_EXPORT LayerTreeImpl {
     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;
@@ -206,6 +209,10 @@ protected:
   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);
 };