[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..8411411538e8f44d12829841af1ae625bd6e8050 100644
--- a/mojo/runner/desktop/launcher_process.cc
+++ b/mojo/runner/desktop/launcher_process.cc
@@ -15,6 +15,7 @@
 #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"
 
@@ -87,6 +88,8 @@ 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 {
+    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