[Startup Tracing] Enable startup tracing from config file

mojo/runner/desktop/launcher_process.cc

Index: mojo/runner/desktop/launcher_process.cc
diff --git a/mojo/runner/desktop/launcher_process.cc b/mojo/runner/desktop/launcher_process.cc
index 361320ef41f6c57981f0b17f2f48817243c36b8a..6fe08075c7e810382168599622eeb648c1dae84d 100644
--- a/mojo/runner/desktop/launcher_process.cc
+++ b/mojo/runner/desktop/launcher_process.cc
@@ -87,6 +87,8 @@ int LauncherProcessMain(int argc, char** argv) {
         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