Re-add snap-back behavior (Chrome side)

content/renderer/webscrollbarbehavior_impl_mac.h

+// Copyright 2014 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 CONTENT_CHILD_WEBSCROLLBARBEHAVIOR_IMPL_MAC_H_
+#define CONTENT_CHILD_WEBSCROLLBARBEHAVIOR_IMPL_MAC_H_
+
+#include "third_party/WebKit/public/platform/WebScrollbarBehavior.h"
+
+namespace content {
+
+class WebScrollbarBehaviorImpl : public blink::WebScrollbarBehavior {
+ public:
+  WebScrollbarBehaviorImpl();
+
+  virtual bool shouldCenterOnThumb(
+      blink::WebScrollbarBehavior::Button mouseButton,
+      bool shiftKeyPressed,
+      bool altKeyPressed);
+
+  void set_jump_on_track_click(bool jump_on_track_click) {
+    jump_on_track_click_ = jump_on_track_click;
+  }
+
+ private:
+  // The current value of AppleScrollerPagingBehavior from NSUserDefaults.
+  bool jump_on_track_click_;
+};
+
+}  // namespace content
+
+#endif  // CONTENT_CHILD_WEBSCROLLBARBEHAVIOR_IMPL_MAC_H_