Add LayerTreeImpl end/begin/rend/rbegin

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 87c1429ce08904dc8770b8172a4e01b723c4f8f6..7ceff0811ce0d0e01dbf01f2fb16b2f94ea8d7a6 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -17,6 +17,7 @@
 #include "cc/input/event_listener_properties.h"
 #include "cc/input/layer_selection_bound.h"
 #include "cc/layers/layer_impl.h"
+#include "cc/layers/layer_list_iterator.h"
 #include "cc/output/begin_frame_args.h"
 #include "cc/output/renderer.h"
 #include "cc/output/swap_promise.h"
@@ -142,6 +143,11 @@ class CC_EXPORT LayerTreeImpl {
 
   void PushPropertiesTo(LayerTreeImpl* tree_impl);
 
+  LayerListIterator begin();

[danakj, 2016/03/03 00:35:50]

Can you leave a comment saying what these iterate and in what order?

+  LayerListIterator end();
+  LayerListReverseIterator rbegin();
+  LayerListReverseIterator rend();
+
   // TODO(thakis): Consider marking this CC_EXPORT once we understand
   // http://crbug.com/575700 better.
   struct ElementLayers {