[Tracing] Update sync_id to be string type for clock sync

base/trace_event/tracing_agent.cc

 // Copyright 2015 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.
 
 #include "base/trace_event/tracing_agent.h"
 
 namespace base {
 namespace trace_event {
 
 TracingAgent::~TracingAgent() {}
 
 bool TracingAgent::SupportsExplicitClockSync() {
   return false;
 }
 
 void TracingAgent::RecordClockSyncMarker(
-    int sync_id,
+    const std::string& sync_id,
     const RecordClockSyncMarkerCallback& callback) {
   DCHECK(SupportsExplicitClockSync());
 }
 
 
 }  // namespace trace_event
 }  // namespace base