Implement BackgroundFetchManager::{get, fetch}()

third_party/WebKit/Source/modules/background_fetch/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("background_fetch") {
   sources = [
     "BackgroundFetchManager.cpp",
     "BackgroundFetchManager.h",
+    "BackgroundFetchRegistration.cpp",
+    "BackgroundFetchRegistration.h",
     "ServiceWorkerRegistrationBackgroundFetch.cpp",
     "ServiceWorkerRegistrationBackgroundFetch.h",
   ]
 }