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..96465256e30bb25bb428323704d06ec625215746 100644
--- a/mojo/public/cpp/bindings/binding.h
+++ b/mojo/public/cpp/bindings/binding.h
@@ -193,7 +193,12 @@ class Binding {
 
   // Sets an error handler that will be called if a connection error occurs on
   // the bound message pipe.
+  //
+  // This method may only be called after this Binding has been bound to a
+  // message pipe. The error handler will be reset when this Binding is unbound
+  // or closed.
   void set_connection_error_handler(const Closure& error_handler) {
+    DCHECK(is_bound());
     internal_state_.set_connection_error_handler(error_handler);
   }