GitLab

There are 3 changes make in absolute-position-behind-scrollbar:

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

This should fix the (absolute/fixed)-position-behind-scrollbar.html
flaky on virtual/threaded test suites.

Bug: 841567, 846424
Change-Id: I3744198054fdf7351ffc769ebfafea11044baf4c
Reviewed-on: https://chromium-review.googlesource.com/1072270
Commit-Queue: Jianpeng Chao <chaopeng@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561904}