Extract shell methods from ApplicationImpl into a base class, and pass this to Initialize() instead.

mojo/services/tracing/public/cpp/tracing_impl.h

Index: mojo/services/tracing/public/cpp/tracing_impl.h
diff --git a/mojo/services/tracing/public/cpp/tracing_impl.h b/mojo/services/tracing/public/cpp/tracing_impl.h
index 97944e84aae3d2e4a0970aaa7e91664b56c522fd..97db1434edab70f839bccce5a90940d2294ea62e 100644
--- a/mojo/services/tracing/public/cpp/tracing_impl.h
+++ b/mojo/services/tracing/public/cpp/tracing_impl.h
@@ -13,7 +13,7 @@
 namespace mojo {
 
 class ApplicationConnection;
-class ApplicationImpl;
+class Shell;
 
 // Connects to mojo:tracing during your Application's Initialize() call once
 // per process.
@@ -32,7 +32,7 @@ class TracingImpl : public InterfaceFactory<tracing::TraceProvider> {
 
   // This connects to the tracing service and registers ourselves to provide
   // tracing data on demand.
-  void Initialize(ApplicationImpl* app);
+  void Initialize(Shell* shell, const std::string& url);
 
  private:
   // InterfaceFactory<tracing::TraceProvider> implementation.