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

Requested by alokp@ in the linked bug. This is sometimes useful.

BUG=447847
R=alokp@chromium.org

Review URL: https://codereview.chromium.org/850953005

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 f59b897..0cd4f03 100644
--- a/mojo/public/cpp/bindings/strong_binding.h
+++ b/mojo/public/cpp/bindings/strong_binding.h
@@ -105,6 +105,7 @@
     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(); }