Add a mojo struct corresponding and typemapped to base::File.

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]
@@ skipping 22 matching lines @@
 // 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;
 };
+
+// Corresponds to |base::File| in base/files/file.h
+struct File {
+  handle fd;
+};