Makes the BattOrAgent asynchronous

tools/battor_agent/battor_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.
 
 #include <string>
 
+#include "base/callback_forward.h"
 #include "base/macros.h"
+#include "base/memory/scoped_ptr.h"
 
 namespace battor {
 
 class BattOrAgent {
  public:
   explicit BattOrAgent(const std::string& path);
   virtual ~BattOrAgent();
 
-  // Tells the BattOr to start tracing.
+  // Tells the BattOr (using a best-effort signal) to start tracing.
   void StartTracing();
 
-  // Tells the BattOr to stop tracing and returns the trace contents.
-  void StopTracing(std::string* out_trace);
+  // Tells the BattOr to stop tracing and calls the callback when the
+  // trace output is available.
+  void StopTracing(
+      const base::Callback<void(scoped_ptr<std::string>)>& callback);
 
-  // Tells the BattOr to record a clock sync marker in its own trace log.
+  // Tells the BattOr (using a best-effort signal) to record a clock
+  // sync marker in its own trace log.
   void RecordClockSyncMarker(const std::string& marker);

[Zhen Wang, 2015/10/29 19:40:22]

I think we still need asynchronous call for this because we want to know when
this recording is done.

 
-  // Tells the BattOr to issue clock sync markers to all other tracing
-  // agents that it's connected to.
+  // Tells the BattOr (using a best-effort signal) to issue clock sync
+  // markers to all other tracing agents that it's connected to.
   void IssueClockSyncMarker();
 
   // Returns whether the BattOr is able to record clock sync markers
   // in its own trace log.
   static bool SupportsExplicitClockSync() { return true; }
 
  private:
   std::string path_;
 
   DISALLOW_COPY_AND_ASSIGN(BattOrAgent);
 };
 
 }  // namespace battor