[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>
 
@@ skipping 69 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 {
+    base::trace_event::EnableStartupTracingIfConfigFileExists();
   }
 
   // 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