Bug 1572920 [wpt PR 18369] - Update interfaces/dom.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/dom.idl

Source: https://github.com/tidoust/reffy-reports/blob/3336bd0/whatwg/idl/dom.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/570123391

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wpt-commits: 70c9ba9d0c18220f41eeca6686325414798b1420
wpt-pr: 18369

testing/web-platform/tests/interfaces/dom.idl

@@ -359,7 +359,7 @@ interface Element : Node {
   [CEReactions] Attr? setAttributeNodeNS(Attr attr);
   [CEReactions] Attr removeAttributeNode(Attr attr);
 
-  ShadowRoot attachShadow(optional ShadowRootInit init = {});
+  ShadowRoot attachShadow(ShadowRootInit init);
   readonly attribute ShadowRoot? shadowRoot;
 
   Element? closest(DOMString selectors);