Plumb whether or not a frame is in shadow DOM to the embedder.

public/web/WebTreeScopeType.h

Index: public/web/WebTreeScopeType.h
diff --git a/public/web/WebTreeScopeType.h b/public/web/WebTreeScopeType.h
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+// 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.
+
+#ifndef WebTreeScopeType_h
+#define WebTreeScopeType_h
+
+namespace blink {
+
+// Indicates if an item is in a document tree or in a shadow tree, per the
+// Shadow DOM spec: https://w3c.github.io/webcomponents/spec/shadow/
+enum class WebTreeScopeType {
+    Document,
+    Shadow,
+    Last = Shadow,
+};
+
+} // namespace blink
+
+#endif // WebTreeScopeType_h