Mojo C++ bindings: Simplify associated interface API.

mojo/public/cpp/bindings/interface_request.h

Index: mojo/public/cpp/bindings/interface_request.h
diff --git a/mojo/public/cpp/bindings/interface_request.h b/mojo/public/cpp/bindings/interface_request.h
index db102368b86be2837653e96a9f738e79e237fd90..29d883615e6a8638fa7f89f814d1fa5b5164a67a 100644
--- a/mojo/public/cpp/bindings/interface_request.h
+++ b/mojo/public/cpp/bindings/interface_request.h
@@ -86,12 +86,9 @@ class InterfaceRequest {
     if (!handle_.is_valid())
       return;
 
-    base::Optional<DisconnectReason> reason;
-    reason.emplace(custom_reason, description);
-
     Message message =
         PipeControlMessageProxy::ConstructPeerEndpointClosedMessage(
-            kMasterInterfaceId, reason);
+            kMasterInterfaceId, DisconnectReason(custom_reason, description));
     MojoResult result = WriteMessageNew(
         handle_.get(), message.TakeMojoMessage(), MOJO_WRITE_MESSAGE_FLAG_NONE);
     DCHECK_EQ(MOJO_RESULT_OK, result);