Make the default tabIndex for <frame> 0

This matches all implementations. Fixes #4855.

This is another tweak on top of 15cf23a
and 7ce0b47.

source

@@ -73793,10 +73793,10 @@ END:VCARD</pre>
   <p>The <dfn><code data-x="dom-tabIndex">tabIndex</code></dfn> IDL attribute must
   <span>reflect</span> the value of the <code data-x="attr-tabindex">tabindex</code> content
   attribute. The default value is 0 if the element is an <code>a</code>, <code>area</code>,
-  <code>button</code>, <code>iframe</code>, <code>input</code>, <code>object</code>,
-  <code>select</code>, <code>textarea</code>, or <span>SVG <code>a</code></span> element, or is a
-  <code>summary</code> element that is a <span>summary for its parent details</span>. The default
-  value is &#x2212;1 otherwise.</p>
+  <code>button</code>, <code>frame</code>, <code>iframe</code>, <code>input</code>,
+  <code>object</code>, <code>select</code>, <code>textarea</code>, or <span>SVG
+  <code>a</code></span> element, or is a <code>summary</code> element that is a <span>summary for
+  its parent details</span>. The default value is &#x2212;1 otherwise.</p>
 
   <p class="note">The varying default value based on element type is a historical artifact.</p>