Cleanup: remove non-standard retain_unknown_fields from .proto files

google_apis/gcm/protocol/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.
 //
 // Request and reply to the "checkin server" devices poll every few hours.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
-option retain_unknown_fields = true;
 
 package checkin_proto;
 
 import "android_checkin.proto";
 
 // A concrete name/value pair sent to the device's Gservices database.
 message GservicesSetting {
   required bytes name = 1;
   required bytes value = 2;
 }
@@ skipping 126 matching lines @@
   repeated GservicesSetting setting = 5;
 
   optional bool market_ok = 6;   // If Market got the market_checkin data OK.
 
   optional fixed64 android_id = 7;      // From the request, or newly assigned
   optional fixed64 security_token = 8;  // The associated security token
 
   optional string version_info = 11;
   // NEXT TAG: 12
 }