Add a custom element to own structure of shadow DOM plugin placeholders.

Source/core/html/shadow/PluginPlaceholderElement.idl

Index: Source/core/html/shadow/PluginPlaceholderElement.idl
diff --git a/Source/core/xml/DocumentXMLTreeViewer.idl b/Source/core/html/shadow/PluginPlaceholderElement.idl
similarity index 55%
copy from Source/core/xml/DocumentXMLTreeViewer.idl
copy to Source/core/html/shadow/PluginPlaceholderElement.idl
index 150d440592945c6a749b5c1965ddc0972474bce6..54b5e49aa7d8ae20599405abf212aa296de30b2f 100644
--- a/Source/core/xml/DocumentXMLTreeViewer.idl
+++ b/Source/core/html/shadow/PluginPlaceholderElement.idl
@@ -3,7 +3,8 @@
 // found in the LICENSE file.
 
 [
-    NoImplHeader
-] partial interface Document {
-    [ImplementedInPrivateScript, OnlyExposedToPrivateScript] void transformDocumentToTreeView(DOMString noStyleMessage);
+    NoInterfaceObject
+] interface PluginPlaceholderElement : HTMLDivElement {
+    [ImplementedInPrivateScript, OnlyExposedToPrivateScript] attribute DOMString message;
+    [ImplementedInPrivateScript, OnlyExposedToPrivateScript] void createdCallback();
 };