Add impl() getter to StrongBinding to be consistent with Binding

mojo/public/cpp/bindings/strong_binding.h

Index: mojo/public/cpp/bindings/strong_binding.h
diff --git a/mojo/public/cpp/bindings/strong_binding.h b/mojo/public/cpp/bindings/strong_binding.h
index f59b897b4fd85e2e54859abf24c49a71c92e4a18..0cd4f03beb8838e8018fad6017d6da96b35a634e 100644
--- a/mojo/public/cpp/bindings/strong_binding.h
+++ b/mojo/public/cpp/bindings/strong_binding.h
@@ -105,6 +105,7 @@ class StrongBinding : public ErrorHandler {
     error_handler_ = error_handler;
   }
 
+  Interface* impl() { return binding_.impl(); }
   typename Interface::Client* client() { return binding_.client(); }
   // Exposed for testing, should not generally be used.
   internal::Router* internal_router() { return binding_.internal_router(); }