Convert overflow-scrollability.html to new test style

There are 3 changes make in overflow-scrollability.html:

1. update tests to use testharness.js
2. call rAFs before start input
3. use promise_test and gpuBenchmarking events

This should fix the overflow-scrollability.html flaky on virtual/threaded
test suites.

Bug: 841567
Change-Id: Ic098832a3ce685774d204d245f61809edee4a60b
Reviewed-on: https://chromium-review.googlesource.com/1131834
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Jianpeng Chao <chaopeng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575793}
2 files changed