[Startup Tracing] Enable startup tracing from config file

mojo/runner/desktop/launcher_process.cc

 // Copyright 2013 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 <stdio.h>
 #include <string.h>
 
 #include <algorithm>
 #include <iostream>
 
 #include "base/base_switches.h"
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/files/file_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/trace_event/trace_event.h"
+#include "components/tracing/startup_tracing.h"
 #include "mojo/runner/context.h"
 #include "mojo/runner/switches.h"
 
 namespace mojo {
 namespace runner {
 namespace {
 
 // Whether we're currently tracing.
 bool g_tracing = false;
 
@@ skipping 52 matching lines @@
 int LauncherProcessMain(int argc, char** argv) {
   const base::CommandLine& command_line =
       *base::CommandLine::ForCurrentProcess();
   if (command_line.HasSwitch(switches::kTraceStartup)) {
     g_tracing = true;
     base::trace_event::TraceConfig trace_config(
         command_line.GetSwitchValueASCII(switches::kTraceStartup),
         base::trace_event::RECORD_UNTIL_FULL);
     base::trace_event::TraceLog::GetInstance()->SetEnabled(
         trace_config, base::trace_event::TraceLog::RECORDING_MODE);
+  } else {
+    tracing::EnableStartupTracingIfConfigFileExists();

[yzshen1, 2015/06/16 23:44:58]

I haven't looked at the code very closely. In this case, g_tracing is not
touched, is that expected? For example, StopTracingAndFlushToDisk() is not
called for sure.

[Zhen Wang, 2015/06/16 23:49:00]

Yes. This is expected. So the existing way is to use --trace-startup flag. You
need to specify the time period when trace is running. So it will schedule a
function (StopTracingAndFlushToDisk) to stop tracing automatically after the
specified period.

If using startup tracing via Telemetry, Telemetry will control the config file.
Telemetry puts the config file to the well-known location before the browser
starts. Later, Telemetry will stop tracing via DevTools. Therefore, there is no
need to call StopTracingAndFlushToDisk().

   }
 
   // We want the shell::Context to outlive the MessageLoop so that pipes are
   // all gracefully closed / error-out before we try to shut the Context down.
   Context shell_context;
   {
     base::MessageLoop message_loop;
     if (!shell_context.Init()) {
       return 0;
     }
@@ skipping 12 matching lines @@
     shell_context.Shutdown();
   }
 
   if (g_tracing)
     StopTracingAndFlushToDisk();
   return 0;
 }
 
 }  // namespace runner
 }  // namespace mojo