cc : Update render surfaces using LayerListIterator instead of treewalk

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 074ab1daddf05c99a4a212fc903235839f770066..30add9774e2e572fb10543a3d9c9b963a97890db 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -563,7 +563,8 @@ void LayerTreeImpl::SetPageScaleOnActiveTree(float active_page_scale) {
           property_trees(), PageScaleLayer(), current_page_scale_factor(),
           device_scale_factor(), layer_tree_host_impl_->DrawTransform());
     } else {
-      DCHECK(!root_layer_ || active_page_scale == 1);
+      DCHECK(!root_layer_ || active_page_scale == 1 ||
+             root_layer_->transform_tree_index() == -1);

[jaydasika, 2016/03/22 00:19:07]

have to remove this (added it to make unit tests pass), but I am unsure about
the alternative. I tried building property trees and setting page scale layer
before this function is called to make the tests take the other path (if
statement), but LayerById(page_scale_layer_id) returns null inspite of that (I
think its because of the ownership of these layers, they are scoped_ptr created
in the test). Any thoughts on what to do ?

[ajuma, 2016/03/22 14:41:55]

Could it be that the page_scale_layer_id is invalid? LayerImpls get added to the
map in their constructor and removed in their destructor, so if the id is valid,
it should be case that LayerById(id) returns non-null. If the
page_scale_layer_id is indeed valid, try adding some logging to
LayerTreeImpl::RegisterLayer and ::UnregisterLayer to see if these are behaving
as expected.

     }
   }
 }