Add constructor to InstallEvent (#1729)

Closes #1713.

Author: sisidovski

docs/index.bs

@@ -1563,6 +1563,7 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
     <pre class="idl">
       [Exposed=ServiceWorker]
       interface InstallEvent : ExtendableEvent {
+        constructor(DOMString type, optional ExtendableEventInit eventInitDict = {});
         Promise&lt;undefined&gt; addRoutes((RouterRule or sequence&lt;RouterRule&gt;) rules);
       };