Implement Element.scrollIntoView for scroll-behavior: smooth.

third_party/WebKit/Source/platform/scroll/SmoothScrollSequencer.h

+// Copyright (c) 2017 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.
+#ifndef SmoothScrollSequencer_h
+#define SmoothScrollSequencer_h
+
+#include <utility>
+#include "platform/heap/Handle.h"
+#include "platform/scroll/ScrollTypes.h"
+
+namespace blink {
+
+class ScrollableArea;
+
+// A sequencer that queues the nested scrollers from inside to outside,
+// so that they can be animated from outside to inside when smooth scroll
+// is called.
+class PLATFORM_EXPORT SmoothScrollSequencer final
+    : public GarbageCollected<SmoothScrollSequencer> {
+ public:
+  SmoothScrollSequencer();
+  // Add a scroll offset animation to the back of a queue.
+  void QueueAnimation(ScrollableArea*, ScrollOffset);
+
+  // Run the animation at the back of the queue.
+  void RunQueuedAnimations();
+
+  // Abort the currently running animation and all the animations in the queue.
+  void AbortAnimations();
+
+  DECLARE_TRACE();
+
+ private:
+  typedef std::pair<Member<ScrollableArea>, ScrollOffset> ScrollerAndOffsetPair;
+  HeapVector<ScrollerAndOffsetPair> queue_;
+  Member<ScrollableArea> current_scrollable_;
+};
+
+}  // namespace blink
+
+#endif  // SmoothScrollSequencer_h