Adds Mojo IDL.

third_party/WebKit/Source/core/mojo/MojoWriteMessageOptions.idl

Index: third_party/WebKit/Source/core/mojo/MojoWriteMessageOptions.idl
diff --git a/third_party/WebKit/Source/core/mojo/MojoWriteMessageOptions.idl b/third_party/WebKit/Source/core/mojo/MojoWriteMessageOptions.idl
new file mode 100644
index 0000000000000000000000000000000000000000..25ffc9d8fe3568f569d4ead00d93c266c5a559dc
--- /dev/null
+++ b/third_party/WebKit/Source/core/mojo/MojoWriteMessageOptions.idl
@@ -0,0 +1,8 @@
+// Copyright 2017 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.
+
+dictionary MojoWriteMessageOptions {
+  BufferSource buffer;
+  sequence<MojoHandle> handles;
+};