Makes the BattOrAgent asynchronous

tools/battor_agent/battor_agent.cc

Index: tools/battor_agent/battor_agent.cc
diff --git a/tools/battor_agent/battor_agent.cc b/tools/battor_agent/battor_agent.cc
index 744c61b6db5ae1430260be1880ac18bcc6f0d3a5..da00eecf6a4c41d5e664bca7316b6f2b0dec5b7f 100644
--- a/tools/battor_agent/battor_agent.cc
+++ b/tools/battor_agent/battor_agent.cc
@@ -6,6 +6,9 @@
 
 #include <iostream>
 
+#include "base/callback.h"
+#include "base/memory/scoped_ptr.h"
+
 namespace battor {
 
 BattOrAgent::BattOrAgent(const std::string& path) : path_(path) {
@@ -20,8 +23,10 @@ void BattOrAgent::StartTracing() {
   // TODO: Tell the BattOr to start tracing.
 }
 
-void BattOrAgent::StopTracing(std::string* out_trace) {
+void BattOrAgent::StopTracing(
+    const base::Callback<void(scoped_ptr<std::string>)>& callback) {
   // TODO: Tell the BattOr to stop tracing.
+  callback.Run(scoped_ptr<std::string>(new std::string("battor trace output")));

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

I think this callback run eventually should be moved to somewhere else, right?
For example, after BattOr sends trace data back?

[charliea (OOO until 10-5), 2015/10/29 21:12:11]

Yea. 

This callback will likely be sent in as the callback for SerialIOHandler::Write
(https://code.google.com/p/chromium/codesearch#chromium/src/device/serial/seri...).

 }
 
 void BattOrAgent::RecordClockSyncMarker(const std::string& marker) {