Remove a setTimeout()'s entry in the map of active timers after it runs

It used to be unobservable whether a non-repeating timer that had already run its callback had an entry in the list of active timers or not, and so those entries were never removed from the list. However, calculation of the idle deadline, meaning that if a timer expires and is never cancelled, the deadline will be in the past.

This change avoids this case by removing non-repeating timers from the map of active timers after their callback runs.

Closes whatwg#7376.

source

@@ -96875,6 +96875,9 @@ enum <dfn enum>DOMParserSupportedType</dfn> {
      <li><p>If <var>repeat</var> is true, then perform the <span>timer initialization
      steps</span> again, given <var>global</var>, <var>handler</var>, <var>timeout</var>,
      <var>arguments</var>, true, and <var>id</var>.</p></li>
+
+     <li><p>Otherwise, <span data-x="map remove">remove</span> <var>global</var>'s <span>map of
+     active timers</span>[<var>id</var>].</p></li>
     </ol>
    </li>
 
@@ -96886,16 +96889,9 @@ enum <dfn enum>DOMParserSupportedType</dfn> {
    task">queues a global task</span> on the <dfn export>timer task source</dfn> given
    <var>global</var> to run <var>task</var>.</p></li>
 
-   <li>
-    <p><span>Run steps after a timeout</span> given <var>global</var>, "<code
-    data-x="">setTimeout/setInterval</code>", <var>timeout</var>, <var>completionStep</var>, and
-    <var>id</var>.</p>
-
-    <p class="note">Once the task has been processed, if <var>repeat</var> is false, it is safe to
-    remove the entry for <var>id</var> from the <span>map of active timers</span> (there is no way
-    for the entry's existence to be detected past this point, so it does not technically matter one
-    way or the other).</p>
-   </li>
+   <li><p><span>Run steps after a timeout</span> given <var>global</var>, "<code
+   data-x="">setTimeout/setInterval</code>", <var>timeout</var>, <var>completionStep</var>, and
+   <var>id</var>.</p></li>
 
    <li><p>Return <var>id</var>.</p></li>
   </ol>
@@ -96996,6 +96992,9 @@ scheduleWork(); // queues a task to do lots of work</code></pre>
      </li>
 
      <li><p>Perform <var>completionSteps</var>.</p></li>
+
+     <li><p>If <var>timerKey</var> is a non-numeric value, <span data-x="map remove">remove</span>
+     <var>global</var>'s <span>map of active timers</span>[<var>timerKey</var>].</p></li>
     </ol>
    </li>
   </ol>