Hide all popovers when beforetoggle shows a popover

See http://crrev.com/c/5141430:

It was previously possible to hit some underspecified behavior with something like this:

<div popover id=p1>Popover 1
  <div popover id=p2>Popover 2</div>
</div>
<script>
  p1.showPopover();
  p1.addEventListener('beforetoggle',() => p2.showPopover());
  p1.hidePopover();
</script>

The problem is that "hide all popovers until" doesn't end up with the desired "until" popover on the top of the stack in this case. There is already a similar situation within the "hide all..." algorithm itself, but that only handles the case where a popover being hidden by "hide all..." has the beforetoggle listener. This is the same problem, but for the case that the "until" popover has that listener.

source

@@ -84611,6 +84611,10 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
     </ol>
    </li>
 
+   <li><p>Let <var>autoPopoverListContainsElement</var> be true if <var>document</var>'s
+   <span>showing auto popover list</span>'s last item is <var>element</var>, otherwise
+   false.</p></li>
+
    <li><p>Set <var>element</var>'s <span>popover invoker</span> to null.</p></li>
 
    <li>
@@ -84623,6 +84627,11 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
      data-x="">open</code>" and the <code data-x="dom-ToggleEvent-newState">newState</code>
      attribute initialized to "<code data-x="">closed</code>" at <var>element</var>.</p></li>
 
+     <li><p>If <var>autoPopoverListContainsElement</var> is true and <var>document</var>'s
+     <span>showing auto popover list</span>'s last item is not <var>element</var>, then run <span
+     data-x="hide-all-popovers-until">hide all popovers until</span> given <var>element</var>,
+     <var>focusPreviousElement</var>, and false.</p></li>
+
      <li>
       <p>If the result of running <span>check popover validity</span> given <var>element</var>,
       true, <var>throwExceptions</var>, and null is false, then run <var>cleanupSteps</var> and
@@ -84702,6 +84711,10 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
   <var>focusPreviousElement</var>, and a boolean <var>fireEvents</var>:</p>
 
   <ol>
+   <li><p>If <var>endpoint</var> is an <span data-x="HTML elements">HTML element</span> and
+   <var>endpoint</var> is not in the <span data-x="popover-showing-state">popover showing
+   state</span>, then return.</p></li>
+
    <li><p>Let <var>document</var> be <var>endpoint</var>'s <span>node document</span>.</p></li>
 
    <li>