[Startup Tracing] Enable startup tracing from config file

mandoline/app/desktop/launcher_process.cc

Index: mandoline/app/desktop/launcher_process.cc
diff --git a/mandoline/app/desktop/launcher_process.cc b/mandoline/app/desktop/launcher_process.cc
index 7f99d7c8c8c3bee44a159597eab67bc5b8d552c6..95e876b8fe4d2388dd0b1b486c373fbf75293fae 100644
--- a/mandoline/app/desktop/launcher_process.cc
+++ b/mandoline/app/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 "mandoline/app/core_services_initialization.h"
 #include "mandoline/app/desktop/launcher_process.h"
 #include "mojo/runner/context.h"
@@ -88,6 +89,10 @@ 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 {
+    // |g_tracing| is not touched in this case and Telemetry will stop tracing
+    // on demand later.
+    tracing::EnableStartupTracingIfConfigFileExists();

[sky, 2015/06/18 22:18:16]

Don't you need to update BUILD.gn for the new dep?

   }
 
   // We want the runner::Context to outlive the MessageLoop so that pipes are