BroadcastChannel: Ignore msgs from detached iframes/closing workers

For BroadcastChannel instances associated with detached iframes or closing workers, update the specification to require that calls to postMessage() should be ignored (without throwing an exception).

Closes whatwg#7219. Some potential followup in whatwg#7253 for the related bfcache case.

source

@@ -101394,11 +101394,25 @@ interface <dfn interface>BroadcastChannel</dfn> : <span>EventTarget</span> {
   data-x="dom-BroadcastChannel-name">name</code></dfn> getter steps are to return
   <span>this</span>'s <span>channel name</span>.</p>
 
+  <p>A <code>BroadcastChannel</code> object is said to be <dfn>eligible for messaging</dfn> when
+  its <span>relevant global object</span> is either:</p>
+
+  <ul>
+   <li><p>a <code>Window</code> object whose <span data-x="concept-document-window">associated
+   <code>Document</code></span> is <span>fully active</span>, or</p></li>
+
+   <li><p>a <code>WorkerGlobalScope</code> object whose <span
+   data-x="dom-WorkerGlobalScope-closing">closing</span> flag is false and whose
+   <span>worker</span> is not a <span>suspendable worker</span>.</p></li>
+  </ul>
+
   <p>The <dfn method for="BroadcastChannel"><code
   data-x="dom-BroadcastChannel-postMessage">postMessage(<var>message</var>)</code></dfn> method
   steps are:</p>
 
   <ol>
+   <li><p>If <span>this</span> is not <span>eligible for messaging</span>, then return.</p></li>
+
    <li><p>If <span>this</span>'s <span data-x="concept-BroadcastChannel-closed">closed flag</span>
    is true, then throw an <span>"<code>InvalidStateError</code>"</span>
    <code>DOMException</code>.</p></li>
@@ -101414,18 +101428,7 @@ interface <dfn interface>BroadcastChannel</dfn> : <span>EventTarget</span> {
     match the following criteria:</p>
 
     <ul>
-     <li>
-      <p>Their <span>relevant global object</span> is either:</p>
-
-      <ul>
-       <li><p>a <code>Window</code> object whose <span data-x="concept-document-window">associated
-       <code>Document</code></span> is <span>fully active</span>, or</p></li>
-
-       <li><p>a <code>WorkerGlobalScope</code> object whose <span
-       data-x="dom-WorkerGlobalScope-closing">closing</span> flag is false and whose
-       <span>worker</span> is not a <span>suspendable worker</span>.</p></li>
-      </ul>
-     </li>
+     <li><p>They are <span>eligible for messaging</span>.</p></li>
 
      <li><p>Their <span>relevant settings object</span>'s <span
      data-x="concept-settings-object-origin">origin</span> is <span>same origin</span> with
@@ -126731,6 +126734,7 @@ INSERT INTERFACES HERE
   Andrew Simons,
   Andrew Smith,
   Andrew W. Hagen,
+  Andrew Williams,
   Andrey V. Lukyanov,
   Andry Rendy,
   Andy Davies,