[Sync] Create UserEventSyncBridge.

components/sync/protocol/user_event_specifics.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.
 //
 // Sync protocol datatype extension for user events.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package sync_pb;
 
 message UserEventSpecifics {
   // Time of event, as measured on the client (unix epoch).
   optional int64 event_time_usec = 1;
 
   // The |global_id| field of the associated navigation, if there is one.
   optional int64 navigation_id = 2;
 
+  // On startup we generate a new random id to identify which FieldTrialEvents
+  // other events belonged to.
+  optional fixed64 session_id = 3;
+
   // TODO(skym): Wrap actual event payloads in a oneof.
 }