Cleanup: remove non-standard retain_unknown_fields from .proto files

google_apis/gcm/protocol/android_checkin.proto

 // Copyright 2014 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.
 //
 // Logging information for Android "checkin" events (automatic, periodic
 // requests made by Android devices to the server).
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
-option retain_unknown_fields = true;
 package checkin_proto;
 
 // Build characteristics unique to the Chrome browser, and Chrome OS
 message ChromeBuildProto {
   enum Platform {
     PLATFORM_WIN = 1;
     PLATFORM_MAC = 2;
     PLATFORM_LINUX = 3;
     PLATFORM_CROS = 4;
     PLATFORM_IOS = 5;
@@ skipping 66 matching lines @@
 
   // Apple IOS device
   DEVICE_IOS_OS = 2;
 
   // Chrome browser - Not Chrome OS.  No hardware records.
   DEVICE_CHROME_BROWSER = 3;
 
   // Chrome OS
   DEVICE_CHROME_OS = 4;
 }