Add IntersectionObserverEntry.intersectionRatio attribute.

third_party/WebKit/LayoutTests/intersection-observer/helper-functions.js

 // Some of the js-test.js boilerplate will add stuff to the top of the document early
 // enough to screw with frame offsets that are measured by the test.  Delay all that
 // jazz until the actual test code is finished.
 setPrintTestResultsLazily();
 self.jsTestIsAsync = true;
 
+function rectArea(rect) {
+  return (rect.left - rect.right) * (rect.bottom - rect.top);
+}
+
 function rectToString(rect) {
   return "[" + rect.left + ", " + rect.right + ", " + rect.top + ", " + rect.bottom + "]";
 }
 
 function entryToString(entry) {
   var ratio = ((entry.intersectionRect.width * entry.intersectionRect.height) /
                (entry.boundingClientRect.width * entry.boundingClientRect.height));
   return (
       "boundingClientRect=" + rectToString(entry.boundingClientRect) + "\n" +
       "intersectionRect=" + rectToString(entry.intersectionRect) + "\n" +
       "visibleRatio=" + ratio + "\n" +
       "rootBounds=" + rectToString(entry.rootBounds) + "\n" +
       "target=" + entry.target + "\n" +
       "time=" + entry.time);
 }
+
+function intersectionRatio(entry) {
+  var targetArea = rectArea(entry.boundingClientRect);
+  if (!targetArea)
+    return 0;
+  return rectArea(entry.intersectionRect) / targetArea;
+}