Avoid ServiceWorker failure on addRoutes() failure

Author: yoshisatoyanagisawa

docs/index.bs

@@ -1612,11 +1612,17 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
         1.  For each |rule| of |rules|:
             1. If running the [=Verify Router Condition=] algorithm with |rule|["{{RouterRule/condition}}"] and |serviceWorker| returns false, return [=a promise rejected with=] a {{TypeError}}.
             1. If |rule|["{{RouterRule/source}}"] is "{{RouterSourceEnum/fetch-event}}" and |serviceWorker|'s [=set of event types to handle=] does not [=set/contain=] {{ServiceWorkerGlobalScope/fetch!!event}}, return [=a promise rejected with=] a {{TypeError}}.
-        1. Let |promise| be a new [=promise=].
-        1. [=ExtendableEvent/Add lifetime promise=] |promise| to |event|.
+        1. Let |lifetimePromise| be a new [=promise=].
+        1. [=ExtendableEvent/Add lifetime promise=] |lifetimePromise| to |event|.
 
             Note: {{InstallEvent/addRoutes(rules)|event.addRoutes(rules)}} extends the lifetime of the event by default as if {{ExtendableEvent/waitUntil()|event.waitUntil(promise)}} is called.
 
+        1. Let |promise| be a new [=promise=].
+        1. [=In parallel=]:
+            1. Upon [=upon fulfillment|fulfillment=] or [=upon rejection|rejection=] of |promise|, resolve |lifetimePromise| with undefined.
+
+                Note: this step is for making |lifetimePromise| always fullfilled to avoid the install event failure.
+
         1. Let |serviceWorkerEventLoop| be the [=current global object=]'s [=event loop=].
         1. [=Queue a task=] to run the following steps on |serviceWorkerEventLoop| using the [=DOM manipulation task source=]:
             1. Let |allRules| be a copy of |serviceWorker|'s [=list of router rules=].