cc: Add a main thread LayerListIterator

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 5e061e5d3e94217241eb6cb3a972edf7c5d4e071..6f47b3e4698a402132cf571b92546244c5c6dc7e 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -154,10 +154,10 @@ class CC_EXPORT LayerTreeImpl {
 
   void PushPropertiesTo(LayerTreeImpl* tree_impl);
 
-  LayerListIterator begin();
-  LayerListIterator end();
-  LayerListReverseIterator rbegin();
-  LayerListReverseIterator rend();
+  LayerListIterator<LayerImpl> begin();
+  LayerListIterator<LayerImpl> end();
+  LayerListReverseIterator<LayerImpl> rbegin();
+  LayerListReverseIterator<LayerImpl> rend();
 
   // TODO(thakis): Consider marking this CC_EXPORT once we understand
   // http://crbug.com/575700 better.