media: Use new wrapper types for media mojo interfaces

media/mojo/interfaces/platform_verification.mojom

 // Copyright 2015 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.
 
 // TODO(xhwang): Move this out of media if it is needed anywhere else.
 module media.mojom;
 
 // Allows authorized services to verify that the underlying platform is trusted.
 // An example of a trusted platform is a Chrome OS device in verified boot mode.
 // This can be used for protected content playback.
 //
 // Input parameters:
 // - |service_id|: the service ID for the |challenge|.
 // - |challenge|: the challenge data.
 //
 // Output parameters:
 // - |success|: whether the platform is successfully verified. If true/false the
 //              following 3 parameters should be non-empty/empty.
 // - |signed_data|: the data signed by the platform.
 // - |signed_data_signature|: the signature of the signed data block.
 // - |platform_key_certificate|: the device specific certificate for the
 //                               requested service.
 interface PlatformVerification {
   ChallengePlatform(string service_id, string challenge) =>
       (bool success,
-       string? signed_data,
-       string? signed_data_signature,
-       string? platform_key_certificate);
+       string signed_data,
+       string signed_data_signature,
+       string platform_key_certificate);
 };