Mac: Add rails for scrolling.

content/browser/renderer_host/input/mouse_wheel_rails_filter_mac.h

+// 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.
+
+#ifndef CONTENT_BROWSER_RENDERER_HOST_INPUT_MOUSE_WHEEL_RAILS_FILTER_MAC_H_
+#define CONTENT_BROWSER_RENDERER_HOST_INPUT_MOUSE_WHEEL_RAILS_FILTER_MAC_H_
+
+#include "content/common/content_export.h"
+#include "third_party/WebKit/public/web/WebInputEvent.h"
+#include "ui/gfx/geometry/vector2d_f.h"
+
+namespace content {
+
+class CONTENT_EXPORT MouseWheelRailsFilterMac {

[jdduke (slow), 2015/03/19 22:27:44]

It would be nice to have a brief comment here explaining the basic functionality
and its inspiration (Safari).

+ public:
+  MouseWheelRailsFilterMac();
+  ~MouseWheelRailsFilterMac();
+  blink::WebInputEvent::RailsMode UpdateRailsMode(
+      const blink::WebMouseWheelEvent& event);
+
+ private:
+  gfx::Vector2dF decayed_delta_;

[jdduke (slow), 2015/03/19 22:27:44]

DISALLOW_COPY_AND_ASSIGN

+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_RENDERER_HOST_INPUT_MOUSE_WHEEL_RAILS_FILTER_MAC_H_