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

Source/core/loader/EmptyClients.h

Index: Source/core/loader/EmptyClients.h
diff --git a/Source/core/loader/EmptyClients.h b/Source/core/loader/EmptyClients.h
index 2468785800c56bb8b4559d651c3e9dd2c081d6ef..d04a8561202a6b5b876fb7d647bf0c4770e23f96 100644
--- a/Source/core/loader/EmptyClients.h
+++ b/Source/core/loader/EmptyClients.h
@@ -163,6 +163,8 @@ public:
 
     virtual bool hasWebView() const override { return true; } // mainly for assertions
 
+    virtual bool inShadowTree() const override { return false; }
+
     virtual Frame* opener() const override { return 0; }
     virtual void setOpener(Frame*) override { }