trace_controller: Makes StopTracing callback ignore empty event strings

base/trace_event/tracing_agent.h

Index: base/trace_event/tracing_agent.h
diff --git a/base/trace_event/tracing_agent.h b/base/trace_event/tracing_agent.h
index f8ca78e92dbd07240210249b89db2ed4ecb21b5a..259ff0773dfeb2f47be6ed719df91fc0af42b410 100644
--- a/base/trace_event/tracing_agent.h
+++ b/base/trace_event/tracing_agent.h
@@ -26,6 +26,8 @@ class TraceConfig;
 // 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,