[Offline Pages] Add a GCMAppHandler for offline page prefetch.

components/offline_pages/content/BUILD.gn

 # Copyright 2015 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.
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
 static_library("content") {
   sources = [
     "prefetch_service_factory.cc",
     "prefetch_service_factory.h",
     "suggested_articles_observer.cc",
     "suggested_articles_observer.h",
   ]
 
   deps = [
     "//base",
+    "//components/gcm_driver",
     "//components/keyed_service/content",
     "//components/ntp_snippets",
     "//components/offline_pages/core",
     "//components/offline_pages/core:switches",
     "//components/offline_pages/core/prefetch",
     "//content/public/browser",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "suggested_articles_observer_unittest.cc",
   ]
 
   deps = [
     ":content",
     "//base",
     "//base/test:test_support",
     "//components/offline_pages/core",
     "//components/offline_pages/core:test_support",
     "//content/test:test_support",
     "//testing/gtest",
     "//url",
   ]
 }