Implement overscroll support for the virtual keyboard.

ui/keyboard/keyboard_controller_observer.h

-// Copyright (c) 2013 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 UI_KEYBOARD_KEYBOARD_CONTROLLER_OBSERVER_H_
-#define UI_KEYBOARD_KEYBOARD_CONTROLLER_OBSERVER_H_
-
-#include "ui/keyboard/keyboard_export.h"
-
-namespace gfx {
-class Rect;
-}
-
-namespace keyboard {
-
-// Observers to the KeyboardController are notified of significant events that
-// occur with the keyboard, such as the bounds or visiility changing.
-class KEYBOARD_EXPORT KeyboardControllerObserver {
- public:
-  // Called when the keyboard bounds or visibility are about to change.
-  virtual void OnKeyboardBoundsChanging(const gfx::Rect& new_bounds) = 0;
-};
-
-}  // namespace keyboard
-
-#endif  // UI_KEYBOARD_KEYBOARD_CONTROLLER_OBSERVER_H_