Adds a callback to TracingAgent::StartAgentTracing()

content/browser/tracing/power_tracing_agent.h

Index: content/browser/tracing/power_tracing_agent.h
diff --git a/content/browser/tracing/power_tracing_agent.h b/content/browser/tracing/power_tracing_agent.h
index 58c6a87d02c865e83630fd6f5f2a1f5d73594a38..4542efaaa42a3ae62a986210b773333e7311feb6 100644
--- a/content/browser/tracing/power_tracing_agent.h
+++ b/content/browser/tracing/power_tracing_agent.h
@@ -28,8 +28,8 @@ class PowerTracingAgent : public base::trace_event::TracingAgent {
   std::string GetTracingAgentName() override;
   std::string GetTraceEventLabel() override;
 
-  bool StartAgentTracing(
-      const base::trace_event::TraceConfig& trace_config) override;
+  void StartAgentTracing(const base::trace_event::TraceConfig& trace_config,
+                         const StartAgentTracingCallback& callback) override;
   void StopAgentTracing(const StopAgentTracingCallback& callback) override;
 
   bool SupportsExplicitClockSync() override;