Add "environment discarding steps" hook

These are run when navigation or worker start fails, and will be used by
w3c/ServiceWorker#1315.

source

@@ -82197,7 +82197,9 @@ interface <dfn>Location</dfn> { // but see also <a href="#the-location-interface
     <p>If any of the following are true, then <span data-x="navigate-ua-inline">display the inline
     content with an appropriate error shown to the user</span>, with the newly created
     <code>Document</code> object's <span>origin</span> set to a new <span
-    data-x="concept-origin-opaque">opaque origin</span>, and return.</p>
+    data-x="concept-origin-opaque">opaque origin</span>, run the <span data-x="environment
+    discarding steps">environment discarding steps</span> for <var>reservedEnvironment</var>, and
+    return.</p>
 
     <ul>
      <li><p><var>response</var> is a network error.</p></li>
@@ -86352,6 +86354,13 @@ interface <dfn>ApplicationCache</dfn> : <span>EventTarget</span> {
    unset.</p></dd>
   </dl>
 
+  <p>Specifications may define <dfn data-export="">environment discarding steps</dfn> for
+  environments. The steps take an <span>environment</span> as input.</p>
+
+  <p class="note">The <span data-x="environment discarding steps">environment discarding
+  steps</span> are run for only a select few environments: the ones that will
+  never become execution ready because, for example, they failed to load.</p>
+
   <p>An <dfn data-export="">environment settings object</dfn> is an <span>environment</span> that
   additionally specifies algorithms for:</p>
 
@@ -97241,11 +97250,17 @@ interface <dfn>SharedWorkerGlobalScope</dfn> : <span>WorkerGlobalScope</span> {
      fetch</span> steps with <var>response</var>.</p></li>
     </ol>
 
-    <p>If the algorithm asynchronously completes with null, <span>queue a task</span> to <span
-    data-x="concept-event-fire">fire an event</span> named <code data-x="event-error">error</code>
-    at <var>worker</var>, and return. Otherwise, continue the rest of these steps after
-    the algorithm's asynchronous completion, with <var>script</var> being the asynchronous
-    completion value.</p>
+    <p>If the algorithm asynchronously completes with null, then:</p>
+    <ol>
+    <li><p><span>Queue a task</span> to <span data-x="concept-event-fire">fire an event</span> named
+    <code data-x="event-error">error</code> at <var>worker</var>.</p></li>
+    <li><p>Run the <span data-x="environment discarding steps">environment discarding steps</span>
+    for <var>inside settings</var>.</p></li>
+    <li><p>Return.</p></li>
+    </ol>
+
+    <p>Otherwise, continue the rest of these steps after the algorithm's asynchronous completion,
+    with <var>script</var> being the asynchronous completion value.</p>
    </li>
 
    <li><p>Associate <var>worker</var> with <var>worker global scope</var>.</p></li>