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..7b49f6740aafb692da2730ffed568988886ccf5c 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 write the trace output to
+  // the specified location and calls the callback when complete.
+  void StopTracing(std::string* trace_output, const base::Closure& callback);
 
-  // Tells the BattOr to record a clock sync marker in its own trace log.
-  void RecordClockSyncMarker(const std::string& marker);
+  // Tells the BattOr to record a clock sync marker in its own trace
+  // log and calls the callback when complete.
+  void RecordClockSyncMarker(const std::string& marker,
+                             const base::Closure& callback);
 
-  // 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