Add mojo structs for ScopedFD and ScopedHandle.

mojo/common/platform_handle.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]
+// Corresponds to |base::ScopedFD| in base/files/scoped_file.h
+struct FileDescriptor {
+  handle wrapped_handle;
+};
+
+// Corresponds to |base::win::ScopedHandle| in base/win/scoped_handle.h
+struct WindowsHandle {
+  handle wrapped_handle;
+};
+
+union PlatformHandle {
+  FileDescriptor fd;
+  WindowsHandle windows_handle;
+};