Mojom cpp bindings: Enable non-nullable check.

device/serial/serial.mojom

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module device.serial {
 
 struct DeviceInfo {
-  string path;
+  string? path;
   uint16 vendor_id;
   bool has_vendor_id = false;
   uint16 product_id;
   bool has_product_id = false;
-  string display_name;
+  string? display_name;
 };
 
 enum SendError {
   NONE,
   DISCONNECTED,
   PENDING,
   TIMEOUT,
   SYSTEM_ERROR,
 };
 
@@ skipping 61 matching lines @@
   // Creates a |Connection| to |path| with options specified by |options|,
   // returning it via |connection|. This will fail and |connection| will not be
   // usable if |path| does not specify a valid serial device or there is an
   // error connecting to or configuring the connection.
   Connect(string path,
           ConnectionOptions options,
           Connection& connection);
 };
 
 interface Connection {
-  GetInfo() => (ConnectionInfo info);
+  GetInfo() => (ConnectionInfo? info);
   SetOptions(ConnectionOptions options) => (bool success);
   SetControlSignals(HostControlSignals signals) => (bool success);
   GetControlSignals() => (DeviceControlSignals signals);
   Flush() => (bool success);
 };
 
 }