Fix IntersectionObserver overflow: clip on zero-sized elements.

I realized my fix in bug 1769512 (using IsEmpty()) wasn't quite correct
in this case, because IntersectionObserver preserves edge-inclusive
intersections.

Differential Revision: https://phabricator.services.mozilla.com/D147078

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1770729
gecko-commit: 6fb2ab75b12bbd9708b2476aa2c5e520b776ff02
gecko-reviewers: jwatt

intersection-observer/zero-area-element-visible.html

@@ -14,9 +14,14 @@
   width: 0px;
   height: 0px;
 }
+#container {
+  overflow: clip;
+}
 </style>
 
-<div id='target'></div>
+<div id="container">
+  <div id='target'></div>
+</div>
 
 <script>
 var entries = [];