Implemented additive animations for length

Source/core/animation/AnimationStack.h

Index: Source/core/animation/AnimationStack.h
diff --git a/Source/core/animation/AnimationStack.h b/Source/core/animation/AnimationStack.h
index 07009ff80576c80fdb5e235ff4758bb17bcf3813..1c99c6b65bce46f909f86c9c939eaff5005641d7 100644
--- a/Source/core/animation/AnimationStack.h
+++ b/Source/core/animation/AnimationStack.h
@@ -34,6 +34,7 @@
 #include "core/animation/Animation.h"
 #include "core/animation/AnimationEffect.h"
 #include "core/animation/AnimationPlayer.h"
+#include "core/animation/InterpolationPipeline.h"
 #include "core/animation/SampledEffect.h"
 #include "platform/geometry/FloatBox.h"
 #include "wtf/HashSet.h"
@@ -53,7 +54,7 @@ public:
     bool isEmpty() const { return m_effects.isEmpty(); }
     bool affects(CSSPropertyID) const;
     bool hasActiveAnimationsOnCompositor(CSSPropertyID) const;
-    static WillBeHeapHashMap<CSSPropertyID, RefPtrWillBeMember<Interpolation> > activeInterpolations(AnimationStack*, const WillBeHeapVector<RawPtrWillBeMember<InertAnimation> >* newAnimations, const WillBeHeapHashSet<RawPtrWillBeMember<const AnimationPlayer> >* suppressedAnimationPlayers, Animation::Priority, double timelineCurrentTime);
+    static InterpolationPipelineMap activeInterpolations(AnimationStack*, const WillBeHeapVector<RawPtrWillBeMember<InertAnimation>>* newAnimations, const WillBeHeapHashSet<RawPtrWillBeMember<const AnimationPlayer>>* suppressedAnimationPlayers, Animation::Priority, double timelineCurrentTime);
 
     bool getAnimatedBoundingBox(FloatBox&, CSSPropertyID) const;
     void trace(Visitor*);
@@ -61,7 +62,7 @@ public:
 private:
     void simplifyEffects();
     // Effects sorted by priority. Lower priority at the start of the list.
-    WillBeHeapVector<OwnPtrWillBeMember<SampledEffect> > m_effects;
+    WillBeHeapVector<OwnPtrWillBeMember<SampledEffect>> m_effects;
 
     friend class AnimationAnimationStackTest;
 };