Remove implementation of Panels on OSes other than ChromeOS.

chrome/browser/ui/panels/panel_mouse_watcher_observer.h

-// Copyright (c) 2011 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 CHROME_BROWSER_UI_PANELS_PANEL_MOUSE_WATCHER_OBSERVER_H_
-#define CHROME_BROWSER_UI_PANELS_PANEL_MOUSE_WATCHER_OBSERVER_H_
-
-namespace gfx {
-class Point;
-}
-
-// This observer interface should be implemented in order to receive
-// notifications from PanelMouseWatcher when the mouse moves.
-class PanelMouseWatcherObserver {
- public:
-  // Called when the mouse moves.
-  // |mouse_position| is in screen coordinates.
-  virtual void OnMouseMove(const gfx::Point& mouse_position) = 0;
-
- protected:
-  virtual ~PanelMouseWatcherObserver() {}
-};
-#endif  // CHROME_BROWSER_UI_PANELS_PANEL_MOUSE_WATCHER_OBSERVER_H_