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

components/offline_pages/content/prefetch/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.
+
+if (is_android) {
+  import("//build/config/android/rules.gni")
+}
+
+source_set("prefetch") {
+  sources = [
+    "prefetch_gcm_app_handler.cc",
+    "prefetch_gcm_app_handler.h",
+  ]
+
+  deps = [
+    "//base",
+    "//chrome/browser/android",
+    "//components/gcm_driver",
+    "//content/public/browser",
+  ]
+}
+
+source_set("unit_tests") {
+  testonly = true
+  sources = []
+  deps = [
+    ":prefetch",
+    "//testing/gtest",
+  ]
+}