Make assignedSlot return null for a slot in a closed shadow tree

third_party/WebKit/LayoutTests/fast/dom/shadow/resources/shadow-dom.js

Index: third_party/WebKit/LayoutTests/fast/dom/shadow/resources/shadow-dom.js
diff --git a/third_party/WebKit/LayoutTests/fast/dom/shadow/resources/shadow-dom.js b/third_party/WebKit/LayoutTests/fast/dom/shadow/resources/shadow-dom.js
index 5f3d1c0c0e02a57c002b2d95f17230510fa32a1a..05d1586c5f9ec6b7d5dc33951ff301c0999aad0b 100644
--- a/third_party/WebKit/LayoutTests/fast/dom/shadow/resources/shadow-dom.js
+++ b/third_party/WebKit/LayoutTests/fast/dom/shadow/resources/shadow-dom.js
@@ -95,6 +95,9 @@ function convertTemplatesToShadowRootsWithin(node) {
         } else {
             shadowRoot = parent.attachShadow({'mode': mode});
         }
+        var expose = template.getAttribute("data-expose-as");
+        if (expose)
+            window[expose] = shadowRoot;
         if (template.id)
             shadowRoot.id = template.id;
         var fragments = document.importNode(template.content, true);