cc: Move SyncedScrollOffset to scroll tree

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index 1332f54efe87fc6caae79477c68a8321d5142a15..323c807f627fae0da812190538b5a52b37988fbc 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -129,8 +129,10 @@ class CC_EXPORT LayerTreeImpl {
   void SetRootLayer(scoped_ptr<LayerImpl>);
   scoped_ptr<LayerImpl> DetachLayerTree();
 
-  void SetPropertyTrees(const PropertyTrees& property_trees) {
+  void SetPropertyTrees(const PropertyTrees property_trees) {
     property_trees_ = property_trees;
+    property_trees_.is_main_thread = false;
+    property_trees_.is_active = IsActiveTree();
     property_trees_.transform_tree.set_source_to_parent_updates_allowed(false);
   }
   PropertyTrees* property_trees() { return &property_trees_; }
@@ -191,6 +193,12 @@ class CC_EXPORT LayerTreeImpl {
   }
 
   void UpdatePropertyTreeScrollingAndAnimationFromMainThread();
+  void UpdatePropertyTreeScrollOffset(PropertyTrees* property_trees) {
+    if (root_layer())
+      property_trees_.scroll_tree.UpdateScrollOffsetMap(
+          &property_trees->scroll_tree.scroll_offset_map(),
+          root_layer()->layer_list_impl());

[ajuma, 2016/03/01 22:54:20]

Can this use layer_list_ directly (without going through root_layer() or
depending on the existence of root_layer())?

[sunxd, 2016/03/02 17:45:34]

Since the layer_list change is reverted. I can keep using layer_tree_impl here.
So the problem is resolved currently.

+  }
   void SetPageScaleOnActiveTree(float active_page_scale);
   void PushPageScaleFromMainThread(float page_scale_factor,
                                    float min_page_scale_factor,