Worker: Add missing files in BUILD.gn

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

 # Copyright 2016 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("serviceworkers") {
   sources = [
     "ExtendableEvent.cpp",
     "ExtendableEvent.h",
@@ skipping 39 matching lines @@
     "ServiceWorkerRegistration.h",
     "ServiceWorkerScriptCachedMetadataHandler.cpp",
     "ServiceWorkerScriptCachedMetadataHandler.h",
     "ServiceWorkerThread.cpp",
     "ServiceWorkerThread.h",
     "ServiceWorkerWindowClient.cpp",
     "ServiceWorkerWindowClient.h",
     "ServiceWorkerWindowClientCallback.cpp",
     "ServiceWorkerWindowClientCallback.h",
     "WaitUntilObserver.cpp",
+    "WaitUntilObserver.h",
   ]
 }