Avoid executing the code in the All Fetch Listeners Are Empty algorithm. (#1676)

* Avoid executing the code in the All Fetch Listeners Are Empty algorithm.

* Removed redundant check.

* Add a note that clarify that we ignores handleEvent.

* Updated the note as suggested.

Author: yoshisatoyanagisawa

docs/index.bs

@@ -3009,11 +3009,11 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
           1. Let |callback| be null.
           1. If |eventHandler| is not null and |eventListenerCallback| equals |eventHandler|'s [=event handler/listener=]'s [=event listener/callback=], then set |callback| to the result of [=convert to an ECMAScript value|converting to an ECMAScript value=] |eventHandler|'s [=event handler/value=].
           1. Otherwise, set |callback| to the result of [=convert to an ECMAScript value|converting to an ECMAScript value=] |eventListenerCallback|.
-          1. If [$IsCallable$](|callback|) is false:
-              1. Let |getResult| be [=Completion=]([$Get$](|callback|), <code>handleEvent</code>).
-              1. If |getResult| is [=abrupt completion=], then return false.
-              1. Set |callback| to |getResult|.
-          1. If [$IsCallable$](|callback|) is false, or |callback|'s [=function body=] is not empty (i.e. either a [=statement=] or [=declaration=] exist), then return false.
+          1. If [$IsCallable$](|callback|) is false, then return false.
+
+              Note: [=Event listener/Callback=] objects that use {{handleEvent}} are assumed to be non-empty. This avoids calling the {{handleEvent}} getters, which could modify the event listeners during this check.
+
+          1. If |callback|'s [=function body=] is not empty (i.e. either a [=statement=] or [=declaration=] exist), then return false.
 
           Note: This detects "<code>fetch</code>" listeners like `() => {}`. Some sites have a fetch event listener with empty body to make them recognized by Chromium as a progressive web application (PWA).