cc: Add a main thread LayerListIterator

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index e8bbf5b11fe712f3dd7fa083feec1c137a5315f7..ad95ed543b1abdd1bba7d1f3b70bbac10fae34f4 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -31,6 +31,7 @@
 #include "cc/input/scrollbar.h"
 #include "cc/input/top_controls_state.h"
 #include "cc/layers/layer_collections.h"
+#include "cc/layers/layer_list_iterator.h"
 #include "cc/output/output_surface.h"
 #include "cc/output/renderer_capabilities.h"
 #include "cc/output/swap_promise.h"
@@ -148,6 +149,11 @@ class CC_EXPORT LayerTreeHost : public MutatorHostClient {
   void DidCompleteSwapBuffers() { client_->DidCompleteSwapBuffers(); }
   bool UpdateLayers();
 
+  LayerListIterator<Layer> begin();
+  LayerListIterator<Layer> end();
+  LayerListReverseIterator<Layer> rbegin();
+  LayerListReverseIterator<Layer> rend();
+
   // Called when the compositor completed page scale animation.
   void DidCompletePageScaleAnimation();