Move MediaControlsWindowEventListener to modules/media_controls/.

third_party/WebKit/Source/modules/media_controls/BUILD.gn

 # Copyright 2017 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.
 
 import("//third_party/WebKit/Source/modules/modules.gni")
 
 blink_modules_sources("media_controls") {
   sources = [
     "MediaControlsImpl.cpp",
     "MediaControlsImpl.h",
     "MediaControlsOrientationLockDelegate.cpp",
     "MediaControlsOrientationLockDelegate.h",
+    "MediaControlsWindowEventListener.cpp",
+    "MediaControlsWindowEventListener.h",
   ]
 }