Remove "API proposal for chrome.app.window to intercept all keys."

chrome/browser/ui/views/apps/keyboard_hook_handler.cc

-// Copyright 2014 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.
-
-#include "chrome/browser/ui/views/apps/keyboard_hook_handler.h"
-
-#include "base/memory/singleton.h"
-
-// TODO(sriramsr): Implement for other platforms.
-// Empty implementation for other platforms.
-void KeyboardHookHandler::Register(views::Widget* widget) {
-}
-
-void KeyboardHookHandler::Deregister(views::Widget* widget) {
-}
-
-// static
-KeyboardHookHandler* KeyboardHookHandler::GetInstance() {
-  return Singleton<KeyboardHookHandler,
-                   DefaultSingletonTraits<KeyboardHookHandler>>::get();
-}