Web Animations: Introduce ActiveAnimations and AnimationStack

Source/core/animation/Player.cpp

Index: Source/core/animation/Player.cpp
diff --git a/Source/core/animation/Player.cpp b/Source/core/animation/Player.cpp
index 2cdefe0b60d73b34f9fdc2f6a940b2d082f61f98..abb23f92de91c87fcefd054a7b02b241a2688732 100644
--- a/Source/core/animation/Player.cpp
+++ b/Source/core/animation/Player.cpp
@@ -52,9 +52,17 @@ Player::Player(DocumentTimeline* timeline, TimedItem* content)
     , m_timeline(timeline)
 {
     ASSERT(m_startTime >= 0);
+    if (m_content)
+        m_content->attach(this);
     update();
 }
 
+Player::~Player()
+{
+    if (m_content)
+        m_content->detach();
+}
+
 double Player::currentTimeBeforeDrift() const
 {
     return (effectiveTime(m_timeline->currentTime()) - m_startTime) * m_playbackRate;