ElementAnimations should hold an ObservableList of AnimationPlayers.

cc/test/animation_timelines_test_common.cc

Index: cc/test/animation_timelines_test_common.cc
diff --git a/cc/test/animation_timelines_test_common.cc b/cc/test/animation_timelines_test_common.cc
index e9377635582c17dcfc4ceeaad7445f4a76c15594..9cb8f9b10381ba2fbc9b029840573a2a9405188a 100644
--- a/cc/test/animation_timelines_test_common.cc
+++ b/cc/test/animation_timelines_test_common.cc
@@ -355,17 +355,17 @@ TestLayer* TestHostClient::FindTestLayer(ElementId element_id,
 }
 
 TestAnimationDelegate::TestAnimationDelegate()
-    : started_(false),
-      finished_(false),
-      aborted_(false),
-      takeover_(false),
+    : started_(0),
+      finished_(0),
+      aborted_(0),
+      takeover_(0),
       start_time_(base::TimeTicks()) {}
 
 void TestAnimationDelegate::NotifyAnimationStarted(
     base::TimeTicks monotonic_time,
     TargetProperty::Type target_property,
     int group) {
-  started_ = true;
+  started_++;
   start_time_ = monotonic_time;
 }
 
@@ -373,14 +373,14 @@ void TestAnimationDelegate::NotifyAnimationFinished(
     base::TimeTicks monotonic_time,
     TargetProperty::Type target_property,
     int group) {
-  finished_ = true;
+  finished_++;
 }
 
 void TestAnimationDelegate::NotifyAnimationAborted(
     base::TimeTicks monotonic_time,
     TargetProperty::Type target_property,
     int group) {
-  aborted_ = true;
+  aborted_++;
 }
 
 void TestAnimationDelegate::NotifyAnimationTakeover(
@@ -388,7 +388,7 @@ void TestAnimationDelegate::NotifyAnimationTakeover(
     TargetProperty::Type target_property,
     double animation_start_time,
     std::unique_ptr<AnimationCurve> curve) {
-  takeover_ = true;
+  takeover_++;
 }
 
 AnimationTimelinesTest::AnimationTimelinesTest()
@@ -495,16 +495,22 @@ 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
+             ? base::ObserverList<AnimationPlayer>::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
+             ? base::ObserverList<AnimationPlayer>::Iterator(
+                   &element_animations->players_list())
+                   .GetNext()
+             : nullptr;
 }
 
 int AnimationTimelinesTest::NextTestLayerId() {