Wrap "activate view transition" with script prepare/clean up

This ensures that we perform a microtask checkpoint (and set the incumbent script) right after resolving/rejecting the ViewTransition promises.

It's necessary as those promises need to be resolved before the other rendering operations, mainly style/layout.

source

@@ -103105,8 +103105,21 @@ location.href = '#foo';</code></pre>
    data-x="dom-PageRevealEvent-viewTransition">viewTransition</code> set to
    <var>transition</var>.</p></li>
 
-   <li><p>If <var>transition</var> is not null, then <span
-   data-x="activate view transition">activate</span> <var>transition</var>.</p></li>
+   <li>
+    <p>If <var>transition</var> is not null, then:</p>
+
+    <ol>
+     <li><p><span>Prepare to run script</span> given <var>document</var>.</p></li>
+
+     <li><p><span data-x="activate view transition">Activate</span> <var>transition</var>.</p></li>
+
+     <li><p><span>Clean up after running script</span> given <var>document</var>.</p></li>
+    </ol>
+
+    <p class="note">Activating a view transition might resolve/reject promises, so by wrapping the
+    activation with prepare/cleanup we ensure those promises are handled before the next rendering
+    step.</p>
+   </li>
   </ol>
 
   <p class="note">Though <code data-x="event-pagereveal">pagereveal</code> is guaranteed to be fired