[cssom-view] scrollingElement should not return the body if it is its…

…elf scrollable. https://lists.w3.org/Archives/Public/www-style/2015Apr/0245.html

cssom-view/Overview.bs

@@ -922,7 +922,7 @@ revision of CSS or HTML.
 <p class=atrisk>The <dfn attribute for=Document>scrollingElement</dfn> attribute, on getting, must run these steps:
 
 1. If the {{Document}} is in <a>quirks mode</a>, follow these substeps:
-    1. If <a>the HTML <code>body</code> element</a> exists, return <a>the HTML <code>body</code> element</a> and abort these steps.
+    1. If <a>the HTML <code>body</code> element</a> exists, and it does not have an associated <a>scrolling box</a>, return <a>the HTML <code>body</code> element</a> and abort these steps.
     1. Return null and abort these steps.
 1. If there is a root element, return the root element and abort these steps.
 1. Return null.