Fix whatwg#458: enable cross-origin <track> and EventSource again

whatwg@7c5555a
forgot to add a conditional to the same-origin fallback flag that
caused mode to always end up as “same-origin” when it was set. So
when integrating with Fetch we temporarily regressed EventSource and
<track> by not letting them fetch across origins.

source

@@ -6763,8 +6763,8 @@ a.setAttribute('href', 'http://example.com/'); // change the content attribute d
    is <span data-x="attr-crossorigin-none">No CORS</span>, and "<code data-x="">cors</code>"
    otherwise.</p></li>
 
-   <li><p>If <i>same-origin fallback flag</i> is set, set <var>mode</var> to "<code
-   data-x="">same-origin</code>".</p></li>
+   <li><p>If <i>same-origin fallback flag</i> is set and <var>mode</var> is "<code
+   data-x="">no-cors</code>", set <var>mode</var> to "<code data-x="">same-origin</code>".</p></li>
 
    <li><p>Let <var>credentialsMode</var> be "<code data-x="">include</code>".</p></li>