cc: Replace LayerIterator with iterator that walks layer list and effect tree

cc/layers/layer_list_iterator.h

Index: cc/layers/layer_list_iterator.h
diff --git a/cc/layers/layer_list_iterator.h b/cc/layers/layer_list_iterator.h
index ff9f186e5eae105dda63206fa68f8eeac1d39c96..da092b951eccb4ff6cdc0a0ddc29767386b77e02 100644
--- a/cc/layers/layer_list_iterator.h
+++ b/cc/layers/layer_list_iterator.h
@@ -12,13 +12,8 @@
 
 namespace cc {
 
-class LayerImpl;
-
-// Unlike LayerIterator and friends, these iterators are not intended to permit
-// traversing the RSLL. Rather, they visit a collection of LayerImpls in
-// stacking order. All recursive walks over the LayerImpl tree should be
-// switched to use these classes instead as the concept of a LayerImpl tree is
-// deprecated.
+// This visits a tree of layers in drawing order. For LayerImpls, this is only
+// useful for tests, since there's no LayerImpl tree outside unit tests.
 template <typename LayerType>
 class CC_EXPORT LayerListIterator {
  public: