Modify bindings to enforce that an error handler callback is only set after binding to a msg pipe.

mojo/public/cpp/bindings/binding.h

Index: mojo/public/cpp/bindings/binding.h
diff --git a/mojo/public/cpp/bindings/binding.h b/mojo/public/cpp/bindings/binding.h
index 09cdb88e77b4ebb4b6d3063b95df0a4b2d6ba7ff..55ea8b11c200cb771a1e05b3b1a496c895dcffd6 100644
--- a/mojo/public/cpp/bindings/binding.h
+++ b/mojo/public/cpp/bindings/binding.h
@@ -194,6 +194,7 @@ class Binding {
   // Sets an error handler that will be called if a connection error occurs on
   // the bound message pipe.
   void set_connection_error_handler(const Closure& error_handler) {
+    DCHECK(is_bound());
     internal_state_.set_connection_error_handler(error_handler);
   }