trace_controller: Makes StopTracing callback ignore empty event strings

base/trace_event/tracing_agent.h

 // 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.
 
 #ifndef BASE_TRACE_EVENT_TRACING_AGENT_H_
 #define BASE_TRACE_EVENT_TRACING_AGENT_H_
 
 #include "base/base_export.h"
 #include "base/callback.h"
 #include "base/memory/ref_counted_memory.h"
 #include "base/values.h"
 
 namespace base {
 
 class TimeTicks;
 
 namespace trace_event {
 
 class TraceConfig;
 
 // A tracing agent is an entity that records its own sort of trace. Each
 // tracing method that produces its own trace log should implement this
 // interface. All tracing agents must only be controlled by TracingController.
 // Some existing examples include TracingControllerImpl for Chrome trace events,
 // DebugDaemonClient for CrOs system trace, EtwSystemEventConsumer for Windows
 // system trace and PowerTracingAgent for BattOr power trace.
 class BASE_EXPORT TracingAgent {
  public:
+  // Passing a null or empty events_str_ptr indicates that no trace data is
+  // available for the specified agent.
   using StopAgentTracingCallback = base::Callback<void(
       const std::string& agent_name,
       const std::string& events_label,
       const scoped_refptr<base::RefCountedString>& events_str_ptr)>;
   using RecordClockSyncMarkerCallback = base::Callback<void(
       int sync_id,
       const TimeTicks& issue_ts,
       const TimeTicks& issue_end_ts)>;
 
   virtual ~TracingAgent();
@@ skipping 44 matching lines @@
   // is true in Chrome because all agent threads' clocks are Chrome clock.
   virtual void RecordClockSyncMarker(
       int sync_id,
       const RecordClockSyncMarkerCallback& callback);
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_TRACING_AGENT_H_