PerformanceTests/Bindings/scroll-top.html should use another DOM attribute

scroll-top.html is intended to test performance of DOM attributes that return small integers.
Due to a recent change to Element.scrollTop, Element.scrollTop no longer returns small integers
(it returns a double value). So this CL updates the performance test so that it uses Node.nodeType.

BUG=371429

Review URL: https://codereview.chromium.org/306783003

git-svn-id: svn://svn.chromium.org/blink/trunk@175021 bbb929c8-8fbe-4397-9dbb-9b2b20218538