Rewrite first-scroll-runs-on-compositor for more robustness.

Currently, first-scroll-runs-on-compositor.html uses ShouldBecomeEqual()
from js-test.js to check the final scroll destination. However, it
checks after 0.5secs, which may not be enough for all the platforms,
making the test flaky.

This patch rewrites the test using testharness.js and only checkes after
the page no longer scrolls or has reached the specified destination,
which would be independent from the platform's performance, making the
test robust.

Bug: 778696
Change-Id: Icb91755a3eaecbb5a76023a303fcf9a27f713665
Reviewed-on: https://chromium-review.googlesource.com/777606


Commit-Queue: Sandra Sun <sunyunjia@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518273}