Add base::UnguessableToken

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;
 
+// 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;
 };