Trace-based benchmarking via a mojo app.

mojo/common/tracing_impl.h

Index: mojo/common/tracing_impl.h
diff --git a/mojo/common/tracing_impl.h b/mojo/common/tracing_impl.h
index a996d5c4b281c424e34893d0f0d271408513a015..6ad14358803733bbc42b3d913146485d16469b5f 100644
--- a/mojo/common/tracing_impl.h
+++ b/mojo/common/tracing_impl.h
@@ -22,11 +22,17 @@ class TracingImpl : public InterfaceFactory<tracing::TraceProvider> {
   // tracing data on demand.
   void Initialize(ApplicationImpl* app);
 
+  void set_tracing_already_started(bool tracing_already_started) {
+    tracing_already_started_ = tracing_already_started;
+  }
+
  private:
   // InterfaceFactory<tracing::TraceProvider> implementation.
   void Create(ApplicationConnection* connection,
               InterfaceRequest<tracing::TraceProvider> request) override;
 
+  bool tracing_already_started_;
+
   DISALLOW_COPY_AND_ASSIGN(TracingImpl);
 };