Add ShadowRootInit dictionary.

Source/core/dom/shadow/ShadowRootInit.idl

Index: Source/core/dom/shadow/ShadowRootInit.idl
diff --git a/Source/core/dom/shadow/ShadowRootInit.idl b/Source/core/dom/shadow/ShadowRootInit.idl
new file mode 100644
index 0000000000000000000000000000000000000000..5cdb3416e0032986214c220b9cdfc92072be9c6d
--- /dev/null
+++ b/Source/core/dom/shadow/ShadowRootInit.idl
@@ -0,0 +1,11 @@
+// Copyright 2015 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.
+
+// Spec: https://w3c.github.io/webcomponents/spec/shadow/#shadowrootinit-dictionary
+
+enum ShadowRootMode { "open", "closed" };
+
+dictionary ShadowRootInit {
+    ShadowRootMode mode;
+};