Bug 1578637 [wpt PR 18823] - Update interfaces/appmanifest.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/appmanifest.idl (#18823)

Source: https://github.com/tidoust/reffy-reports/blob/0567d30/whatwg/idl/appmanifest.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/580566995
--

wpt-commits: 42918ac7947eab570cf0621db50dad1802b62d29
wpt-pr: 18823

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

@@ -3,9 +3,9 @@
 // (https://github.com/tidoust/reffy-reports)
 // Source: Web App Manifest (https://w3c.github.io/manifest/)
 
-[Constructor(DOMString type, optional EventInit eventInitDict = {}),
- Exposed=Window]
+[Exposed=Window]
 interface BeforeInstallPromptEvent : Event {
+  constructor(DOMString type, optional EventInit eventInitDict = {});
   Promise<PromptResponseObject> prompt();
 };