trace_controller: Makes StopTracing callback ignore empty event strings

content/browser/tracing/tracing_controller_impl.cc

 // Copyright (c) 2013 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 "content/browser/tracing/tracing_controller_impl.h"
 
 #include "base/bind.h"
 #include "base/cpu.h"
 #include "base/files/file_util.h"
 #include "base/json/string_escape.h"
 #include "base/macros.h"
@@ skipping 696 matching lines @@
     trace_data_sink_ = NULL;
   }
 }
 
 void TracingControllerImpl::OnEndAgentTracingAcked(
     const std::string& agent_name,
     const std::string& events_label,
     const scoped_refptr<base::RefCountedString>& events_str_ptr) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
-  if (trace_data_sink_.get()) {
+  if (trace_data_sink_.get() && events_str_ptr &&
+      !events_str_ptr->data().empty()) {
     std::string json_string;
     if (agent_name == kETWTracingAgentName) {
       // The Windows kernel events are kept into a JSON format stored as string
       // and must not be escaped.
       json_string = events_str_ptr->data();
     } else {
       json_string = base::GetQuotedJSONString(events_str_ptr->data());
     }
     trace_data_sink_->AddAgentTrace(events_label, json_string);
   }
@@ skipping 408 matching lines @@
   is_monitoring_ = is_monitoring;
 #if !defined(OS_ANDROID)
   for (std::set<TracingUI*>::iterator it = tracing_uis_.begin();
        it != tracing_uis_.end(); it++) {
     (*it)->OnMonitoringStateChanged(is_monitoring);
   }
 #endif
 }
 
 }  // namespace content