CC Animation: Activate/deactivate ElementAnimations on Layer addition/removal.

cc/animation/element_animations.cc

Index: cc/animation/element_animations.cc
diff --git a/cc/animation/element_animations.cc b/cc/animation/element_animations.cc
index 404e9a0aebb44a8530eed346e8f2393334996d20..d6822aedf236311aa1b86ef8bb96d8266ecb31b4 100644
--- a/cc/animation/element_animations.cc
+++ b/cc/animation/element_animations.cc
@@ -89,6 +89,9 @@ void ElementAnimations::ElementRegistered(ElementId element_id,
                                           ElementListType list_type) {
   DCHECK_EQ(element_id_, element_id);
 
+  if (!has_element_in_any_list())
+    UpdateActivation(FORCE_ACTIVATION);
+
   if (list_type == ElementListType::ACTIVE)
     set_has_element_in_active_list(true);
   else
@@ -102,6 +105,9 @@ void ElementAnimations::ElementUnregistered(ElementId element_id,
     set_has_element_in_active_list(false);
   else
     set_has_element_in_pending_list(false);
+
+  if (!has_element_in_any_list())
+    animation_host_->DidDeactivateElementAnimations(this);
 }
 
 void ElementAnimations::AddPlayer(AnimationPlayer* player) {