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

cc/layer_tree_host_impl.h

Index: cc/layer_tree_host_impl.h
diff --git a/cc/layer_tree_host_impl.h b/cc/layer_tree_host_impl.h
index 350bb6140321d3e78417cd5096d61face93d5695..bb7c0cbf6dae0d4e9cc92123e915428e70b220d6 100644
--- a/cc/layer_tree_host_impl.h
+++ b/cc/layer_tree_host_impl.h
@@ -173,6 +173,7 @@ public:
     void checkForCompletedTileUploads();
     scoped_ptr<base::Value> activationStateAsValue() const;
     virtual void activatePendingTreeIfNeeded();
+    void setNeedsFullTreeSync(bool needs) { m_needsFullTreeSync = needs; }
 
     // Shortcuts to layers on the active tree.
     LayerImpl* rootLayer() const;
@@ -320,6 +321,12 @@ private:
     // by the next sync from the main thread.
     scoped_ptr<LayerTreeImpl> m_recycleTree;
 
+    // In impl-side painting mode, this is true when the main thread tree
+    // required a full sync into the pending tree, and so the structure of the
+    // pending tree may no longer match the structure of the active tree. A
+    // full sync should be used when activating the pending tree.
+    bool m_needsFullTreeSync;
+
     bool m_scrollDeltaIsInViewportSpace;
     LayerTreeSettings m_settings;
     LayerTreeDebugState m_debugState;