Implemented additive animations for length

Source/core/animation/InterpolationPipeline.cpp

+// 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 "config.h"
+#include "core/animation/InterpolationPipeline.h"
+
+namespace blink {
+
+void InterpolationPipeline::append(PassRefPtrWillBeRawPtr<InterpolationPipelineStage> stage)
+{
+    RefPtrWillBeRawPtr<InterpolationPipelineStage> newStage = stage;
+
+    if (isEmpty() || newStage->interpolation()->isReplaceOnly()) {
+        m_first = newStage;
+        m_last = newStage;
+        ASSERT(m_first.get() == m_last.get());
+    } else {
+        m_last->setNext(newStage);
+        m_last = newStage;
+    }
+
+    newStage->setNext(nullptr);
+}
+
+}