Makes the BattOrAgent asynchronous

tools/battor_agent/battor_agent.h

Index: tools/battor_agent/battor_agent.h
diff --git a/tools/battor_agent/battor_agent.h b/tools/battor_agent/battor_agent.h
index ced696e0bd2a4772c80a1ab4ee574dfa7ce8c856..fbbad048bdb324d4a749c8df38215f1eb7d5f236 100644
--- a/tools/battor_agent/battor_agent.h
+++ b/tools/battor_agent/battor_agent.h
@@ -4,7 +4,9 @@
 
 #include <string>
 
+#include "base/callback_forward.h"
 #include "base/macros.h"
+#include "base/memory/scoped_ptr.h"
 
 namespace battor {
 
@@ -13,17 +15,20 @@ class BattOrAgent {
   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