Move web modal dialog manager files into the constrained_window component.

extensions/shell/browser/shell_web_contents_modal_dialog_manager.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 "components/web_modal/web_contents_modal_dialog_manager.h"
-
-namespace web_modal {
-
-SingleWebContentsDialogManager*
-WebContentsModalDialogManager::CreateNativeWebModalManager(
-    gfx::NativeWindow dialog,
-    SingleWebContentsDialogManagerDelegate* native_delegate) {
-  // TODO(oshima): Investigate if we need to implement this.
-  NOTREACHED();
-  return NULL;
-}
-
-}  // namespace web_modal