Adjust infinite-scroll-event to pass.

scrollTopMax is not a standardized web feature. The actual
functionality was passing as of:

https://chromium.googlesource.com/chromium/src/+/6a9dd6d26726f346345cf1970c2bc3af3ec1afbd

Bug: 997502
Change-Id: Ic120d09d0e6a9652c195afe0ac7cacece917dd47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1949556
Reviewed-by: Nick Burris <nburris@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721318}

css/css-scroll-anchoring/infinite-scroll-event.tentative.html

@@ -33,6 +33,7 @@
   }));
 });
 window.onload = t.step_func(function() {
-  window.scrollTo(0, document.scrollingElement.scrollTopMax - 200);
+  window.scrollTo(0, document.documentElement.scrollHeight);
+  window.scrollBy(0, -200);
 });
 </script>