Fix "update a style block" early return

The new version matches implementation reality and CSSWG resolution.

The algorithm was also inconsistent, as it looked at whether
the element was in a shadow tree or in the document tree, but it was
only specified to be re-run if the element becomes connected or
disconnected.

The CSSWG discussed this in
w3c/csswg-drafts#3096 (comment)
and http://wpt.live/shadow-dom/ShadowRoot-interface.html tests this.

This also matches closer the definition of <link rel="stylesheet">,
which does use connectedness (though it uses "browsing-context
connected", which is a bit different):
https://html.spec.whatwg.org/#link-type-stylesheet

source

@@ -15102,11 +15102,10 @@ interface <dfn>HTMLStyleElement</dfn> : <span>HTMLElement</span> {
    <li><p>If <var>element</var> has an <span>associated CSS style sheet</span>, <span
    data-x="remove a CSS style sheet">remove the CSS style sheet</span> in question.</p></li>
 
-   <li><p>If <var>element</var>'s <span>root</span> is neither a <span>shadow root</span> nor a
-   <span>document</span>, then return.</p></li>
-   <!-- https://software.hixie.ch/utilities/js/live-dom-viewer/?saved=2740
-        Also, per https://github.com/w3c/webcomponents/issues/56 stylesheets should work in
-        disconnected shadow trees. -->
+   <li><p>If <var>element</var> is not <span>connected</span>, then return.</p></li>
+   <!-- See:
+          https://software.hixie.ch/utilities/js/live-dom-viewer/?saved=2740
+          https://github.com/w3c/csswg-drafts/issues/3096 -->
 
    <li>
     <p>If <var>element</var>'s <code data-x="attr-style-type">type</code> attribute is present and