Migrating MediaSession messages to mojo

mojo/common/common_custom_types.mojom

 // Copyright 2016 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 mojo.common.mojom;
 
 [Native]
 struct FilePath;
 
 [Native]
 struct ListValue;
 
 [Native]
 struct DictionaryValue;
 
 [Native]
 struct Time;
 
 struct TimeDelta {
   int64 microseconds;
 };
 
 [Native]
 struct TimeTicks;
 
 [Native]
 struct String16;
 
+[Native]
+struct NullableString16;

[mlamouri (slow - plz ping), 2016/09/27 08:33:11]

This might need to be reviewed by a mojo person. I wonder if it could be
expressed as `String16?`.

+
 // Corresponds to |base::UnguessableToken| in base/unguessable_token.h
 struct UnguessableToken {
   uint64 high;
   uint64 low;
 };
 
 // Corresponds to |base::Version| in base/version.h
 struct Version {
   array<uint32> components;
 };