Adds a callback to TracingAgent::StartAgentTracing()

content/browser/tracing/power_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 CONTENT_BROWSER_TRACING_POWER_TRACING_AGENT_H_
 #define CONTENT_BROWSER_TRACING_POWER_TRACING_AGENT_H_
 
 #include "base/macros.h"
 #include "base/memory/ref_counted_memory.h"
 #include "base/threading/thread.h"
@@ skipping 10 matching lines @@
 
 class PowerTracingAgent : public base::trace_event::TracingAgent {
  public:
   // Retrieve the singleton instance.
   static PowerTracingAgent* GetInstance();
 
   // base::trace_event::TracingAgent implementation.
   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;
   void RecordClockSyncMarker(
       int sync_id,
       const RecordClockSyncMarkerCallback& callback) override;
 
  private:
   // This allows constructor and destructor to be private and usable only
   // by the Singleton class.
@@ skipping 14 matching lines @@
 
   base::Thread thread_;
   scoped_ptr<BattorPowerTraceProvider> battor_trace_provider_;
 
   DISALLOW_COPY_AND_ASSIGN(PowerTracingAgent);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_TRACING_POWER_TRACING_AGENT_H_