Implement Element.scrollIntoView for scroll-behavior: smooth.

third_party/WebKit/Source/platform/scroll/ProgrammaticScrollAnimatorClient.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 ProgrammaticScrollAnimatorClient_h
+#define ProgrammaticScrollAnimatorClient_h
+
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class PLATFORM_EXPORT ProgrammaticScrollAnimatorClient
+    : public GarbageCollectedFinalized<ProgrammaticScrollAnimatorClient> {
+ public:
+  // Called when ProgrammaticScrollAnimator finishes an animation.
+  virtual void notifyAnimationFinished() = 0;
+
+  virtual ~ProgrammaticScrollAnimatorClient() {}
+
+  DEFINE_INLINE_VIRTUAL_TRACE() {}
+};
+
+}  // namespace blink
+
+#endif  // ProgrammaticScrollAnimatorClient_h