Adds a callback to TracingAgent::StartAgentTracing()

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..7179bb072b4e56002c008d7f3ca4350cf7772a93 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:
+  using StartAgentTracingCallback =
+      base::Callback<void(const std::string& agent_name, bool success)>;
   using StopAgentTracingCallback = base::Callback<void(
       const std::string& agent_name,
       const std::string& events_label,
@@ -51,7 +53,8 @@ class BASE_EXPORT TracingAgent {
   virtual std::string GetTraceEventLabel() = 0;
 
   // Starts tracing on the tracing agent with the trace configuration.
-  virtual bool StartAgentTracing(const TraceConfig& trace_config) = 0;
+  virtual void StartAgentTracing(const TraceConfig& trace_config,
+                                 const StartAgentTracingCallback& callback) = 0;
 
   // Stops tracing on the tracing agent. The trace data will be passed back to
   // the TracingController via the callback.