[BattOr] Reduce StartTracing time by about four seconds.

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 6041a8b311794f8ba7254999d95ffb74ed3f1216..16d9aba1742a649d11528da259393c06ab61e75b 100644
--- a/tools/battor_agent/battor_agent.cc
+++ b/tools/battor_agent/battor_agent.cc
@@ -226,6 +226,9 @@ void BattOrAgent::OnBytesSent(bool success) {
     case Action::SEND_GIT_HASH_REQUEST:
       PerformAction(Action::READ_GIT_HASH);
       return;
+    case Action::RESET:
+      CompleteCommand(BATTOR_ERROR_NONE);
+      return;
     default:
       NOTREACHED();
       return;
@@ -352,6 +355,16 @@ void BattOrAgent::OnMessageRead(bool success,
         // Cancel the next data frame timeout.
         timeout_callback_.Cancel();
         CompleteCommand(BATTOR_ERROR_NONE);
+
+        // Proactively reset the BattOr to speed up the next trace.
+        //
+        // Without this proactive reset, the BattOr will take several seconds
+        // to automatically reset the next time that a trace is started. By
+        // proactively resetting the BattOr now, before the trace is printed by
+        // the agent, the reset is parallelized with the final processing of
+        // this trace by the agent. Thus the BattOr will be ready to start
+        // the next trace immediately.
+        PerformAction(Action::RESET);
         return;
       }
 
@@ -475,6 +488,10 @@ void BattOrAgent::PerformAction(Action action) {
       connection_->ReadMessage(BATTOR_MESSAGE_TYPE_CONTROL_ACK);
       return;
 
+    case Action::RESET:
+      SendControlMessage(BATTOR_CONTROL_MESSAGE_TYPE_RESET, 0, 0);
+      return;
+
     case Action::INVALID:
       NOTREACHED();
       return;