ElementAnimations should hold an ObservableList of AnimationPlayers.

cc/test/animation_timelines_test_common.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/test/animation_timelines_test_common.h"
 
 #include "base/memory/ptr_util.h"
 #include "cc/animation/animation_events.h"
 #include "cc/animation/animation_id_provider.h"
 #include "cc/animation/animation_player.h"
@@ skipping 477 matching lines @@
 
   host_->AnimateLayers(time);
   host_->UpdateAnimationState(true, nullptr);
   host_->SetAnimationEvents(std::move(events));
 }
 
 AnimationPlayer* AnimationTimelinesTest::GetPlayerForElementId(
     ElementId element_id) {
   const scoped_refptr<ElementAnimations> element_animations =
       host_->GetElementAnimationsForElementId(element_id);
-  return element_animations ? element_animations->players_list().head()->value()
-                            : nullptr;
+  return element_animations
+             ? ElementAnimations::PlayersList::Iterator(
+                   &element_animations->players_list())
+                   .GetNext()
+             : nullptr;
 }
 
 AnimationPlayer* AnimationTimelinesTest::GetImplPlayerForLayerId(
     ElementId element_id) {
   const scoped_refptr<ElementAnimations> element_animations =
       host_impl_->GetElementAnimationsForElementId(element_id);
-  return element_animations ? element_animations->players_list().head()->value()
-                            : nullptr;
+  return element_animations
+             ? ElementAnimations::PlayersList::Iterator(
+                   &element_animations->players_list())
+                   .GetNext()
+             : nullptr;
 }
 
 int AnimationTimelinesTest::NextTestLayerId() {
   next_test_layer_id_++;
   return next_test_layer_id_;
 }
 
 }  // namespace cc