[cssom-view-1] Fix isVisible()'s opacity check to look up the tree as well. w3c#6850

Author: tabatkins

cssom-view-1/Overview.bs

@@ -1127,7 +1127,8 @@ Note: The {{DOMRect}} object returned by {{Element/getBoundingClientRect()}} is
         return false.
     1. If the {{IsVisibleOptions/checkOpacity}} dictionary member of |options|
         is true,
-        and the computed value of 'opacity' on |this| is ''0'',
+        and |this|, or a [=shadow-inclusive ancestor=] of |this|,
+        has a computed 'opacity' value of ''0'',
         return false.
     1. If the {{IsVisibleOptions/checkVisibilityCSS}} dictionary member of |options|
         is true,