Fix IntersectionObserverEntry.isIntersecting to match other browsers.

Note that no browser matches the spec (see
w3c/IntersectionObserver#432), but that our
behavior is reasonably close to them. So do this to match them.

Differential Revision: https://phabricator.services.mozilla.com/D76603

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1611204
gecko-commit: 867528d1d35bdec48758f8aa899b0491d7ef10e5
gecko-integration-branch: autoland
gecko-reviewers: mstange

Author: emilio

intersection-observer/isIntersecting-threshold.html

@@ -43,7 +43,8 @@
   assert_equals(entries.length, 2);
   assert_true(entries[1].intersectionRatio >= 0.5 &&
               entries[1].intersectionRatio < 1);
-  assert_equals(entries[1].isIntersecting, true);
+  // See https://github.com/w3c/IntersectionObserver/issues/432
+  assert_equals(entries[1].isIntersecting, false);
   scroller.scrollTop = 100;
 }