[Offline Prefetch] Use proto2 syntax for offline prefetch proto

components/offline_pages/core/prefetch/proto/timestamp.proto

-// 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.
-
-syntax = "proto3";
-option optimize_for = LITE_RUNTIME;
-
-package offline_pages.proto;
-
-message Timestamp {
-  // Represents seconds of UTC time since Unix epoch
-  // 1970-01-01T00:00:00Z. Must be from 0001-01-01T00:00:00Z to
-  // 9999-12-31T23:59:59Z inclusive.
-  int64 seconds = 1;
-
-  // Non-negative fractions of a second at nanosecond resolution. Negative
-  // second values with fractions must still have non-negative nanos values
-  // that count forward in time. Must be from 0 to 999,999,999
-  // inclusive.
-  int32 nanos = 2;
-}