[Offline Prefetch] Use proto2 syntax for offline prefetch proto

components/offline_pages/core/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")
 }
 import("//third_party/protobuf/proto_library.gni")
 
 static_library("prefetch") {
@@ skipping 67 matching lines @@
     "//components/offline_pages/core",
   ]
 }
 
 proto_library("proto") {
   sources = [
     "proto/any.proto",
     "proto/offline_pages.proto",
     "proto/operation.proto",
     "proto/status.proto",
-    "proto/timestamp.proto",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "generate_page_bundle_request_unittest.cc",
     "get_operation_request_unittest.cc",
     "prefetch_dispatcher_impl_unittest.cc",
     "prefetch_gcm_app_handler_unittest.cc",
@@ skipping 11 matching lines @@
     "//components/gcm_driver/instance_id",
     "//components/offline_pages/core",
     "//components/offline_pages/core:switches",
     "//components/offline_pages/core:test_support",
     "//net:test_support",
     "//testing/gmock",
     "//testing/gtest",
     "//url",
   ]
 }