base: Stop passing a non-const ref to ObserverListBase::Iterator.

ui/compositor/layer_animator.cc

Index: ui/compositor/layer_animator.cc
diff --git a/ui/compositor/layer_animator.cc b/ui/compositor/layer_animator.cc
index 611647431d4c5acb61c91dd1ff2be0ffc6d68f46..7bc870300c17efe02b096a9e5f0e45f62dc56b13 100644
--- a/ui/compositor/layer_animator.cc
+++ b/ui/compositor/layer_animator.cc
@@ -801,7 +801,7 @@ void LayerAnimator::GetTargetValue(
 
 void LayerAnimator::OnScheduled(LayerAnimationSequence* sequence) {
   if (observers_.might_have_observers()) {
-    ObserverListBase<LayerAnimationObserver>::Iterator it(observers_);
+    ObserverListBase<LayerAnimationObserver>::Iterator it(&observers_);
     LayerAnimationObserver* obs;
     while ((obs = it.GetNext()) != NULL) {
       sequence->AddObserver(obs);