Web Animations CSS: Implement CSS Transitions backed on Web Animations model

Source/core/animation/ActiveAnimations.h

Index: Source/core/animation/ActiveAnimations.h
diff --git a/Source/core/animation/ActiveAnimations.h b/Source/core/animation/ActiveAnimations.h
index b57283b770617b2c8f361da2f06f9f1d6f81d94a..c411b5afeda7b7dbf78331d79249e34a2468c332 100644
--- a/Source/core/animation/ActiveAnimations.h
+++ b/Source/core/animation/ActiveAnimations.h
@@ -44,13 +44,10 @@ public:
     // Animations that are currently active for this element, their effects will be applied
     // during a style recalc.
     AnimationStack* defaultStack() { return &m_defaultStack; }
-    // Tracks the state of active CSS Animations. The individual animations will also be
-    // part of the default stack, but the mapping betwen animation name and player is kept
-    // here.
+    // Tracks the state of active CSS Animations and Transitions. The individual animations
+    // will also be part of the default stack, but the mapping betwen animation name and
+    // player is kept here.
     CSSAnimations* cssAnimations() { return &m_cssAnimations; }
-    // FIXME: Add AnimationStack for CSS Transitions
-    // CSS Transitions form a separate animation stack as they apply at a different level of
-    // the style cascade. Active transitions will not be present in the default stack.
     bool isEmpty() const { return m_defaultStack.isEmpty() && m_cssAnimations.isEmpty(); }
 private:
     AnimationStack m_defaultStack;