Reset virtual time state in scrollbar tests

This prevents ScrollbarTestWithVirtualTimer from polluting global state
for tests following it.

Bug: 791742
Change-Id: Iae3440451833408a6a5bd24b3319b307cd6d3547
Reviewed-on: https://chromium-review.googlesource.com/969582
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544691}