Add IntersectionObserverEntry.intersectionRatio attribute.

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

Index: third_party/WebKit/LayoutTests/intersection-observer/helper-functions.js
diff --git a/third_party/WebKit/LayoutTests/intersection-observer/helper-functions.js b/third_party/WebKit/LayoutTests/intersection-observer/helper-functions.js
index 2aa6ecce92f07bd78915da93bb56ae948c3b19a3..7e2612d49f5806c2244c5d96c295b16c54ef6a02 100644
--- a/third_party/WebKit/LayoutTests/intersection-observer/helper-functions.js
+++ b/third_party/WebKit/LayoutTests/intersection-observer/helper-functions.js
@@ -4,6 +4,10 @@
 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 + "]";
 }
@@ -19,3 +23,10 @@ function entryToString(entry) {
       "target=" + entry.target + "\n" +
       "time=" + entry.time);
 }
+
+function intersectionRatio(entry) {
+  var targetArea = rectArea(entry.boundingClientRect);
+  if (!targetArea)
+    return 0;
+  return rectArea(entry.intersectionRect) / targetArea;
+}