Extract content dependency from keyboard code

ui/keyboard/keyboard_constants.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_CONSTANTS_H_
-#define UI_KEYBOARD_KEYBOARD_CONSTANTS_H_
-
-#include "ui/keyboard/keyboard_export.h"
-
-namespace keyboard {
-
-// The URL of the keyboard extension.
-KEYBOARD_EXPORT extern const char kKeyboardURL[];
-
-// The host of the keyboard extension URL.
-KEYBOARD_EXPORT extern const char kKeyboardHost[];
-
-}  // namespace keyboard
-
-#endif  //  UI_KEYBOARD_KEYBOARD_CONSTANTS_H_