cc: Move SyncedScrollOffset to scroll tree

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 16b7401edfea6f07e80cd1c7c777c56e4ccf9d4a..a5d65ca17724d2559db69ebde57046672fea444b 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -3240,17 +3240,9 @@ static void CollectScrollDeltas(ScrollAndScaleSet* scroll_info,
   if (!layer_impl)
     return;
 
-  gfx::ScrollOffset scroll_delta = layer_impl->PullDeltaForMainThread();
-
-  if (!scroll_delta.IsZero()) {
-    LayerTreeHostCommon::ScrollUpdateInfo scroll;
-    scroll.layer_id = layer_impl->id();
-    scroll.scroll_delta = gfx::Vector2d(scroll_delta.x(), scroll_delta.y());
-    scroll_info->scrolls.push_back(scroll);
-  }
-
-  for (size_t i = 0; i < layer_impl->children().size(); ++i)
-    CollectScrollDeltas(scroll_info, layer_impl->children()[i].get());
+  return layer_impl->layer_tree_impl()
+      ->property_trees()
+      ->scroll_tree.CollectScrollDeltas(scroll_info);
 }
 
 scoped_ptr<ScrollAndScaleSet> LayerTreeHostImpl::ProcessScrollDeltas() {