Implement overscroll support for the virtual keyboard.

ui/keyboard/keyboard_switches.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_SWITCHES_H_
 #define UI_KEYBOARD_KEYBOARD_SWITCHES_H_
 
 #include "ui/keyboard/keyboard_export.h"
 
 namespace keyboard {
 namespace switches {
 
+// Disabled overscrolling of web content when the virtual keyboard is displayed.
+// If disabled, the work area is resized to restrict windows from overlapping
+// with the keybaord area.
+KEYBOARD_EXPORT extern const char kDisableVirtualKeyboardOverscroll[];
+
 // Enables the swipe selection feature on the virtual keyboard.
 KEYBOARD_EXPORT extern const char kEnableSwipeSelection[];
 
 // Enables the virtual keyboard.
 KEYBOARD_EXPORT extern const char kEnableVirtualKeyboard[];
 
+// Enable overscrolling of web content when the virtual keyboard is displayed
+// to provide access to content that would otherwise be occluded.
+KEYBOARD_EXPORT extern const char kEnableVirtualKeyboardOverscroll[];
+
 // Enables the keyboard usability experiment. This is an experimental mode for
 // testing the usability of various experimental keyboard layouts.
 KEYBOARD_EXPORT extern const char kKeyboardUsabilityExperiment[];
 
 }  // namespace switches
 }  // namespace keyboard
 
 #endif  //  UI_KEYBOARD_KEYBOARD_SWITCHES_H_