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This was previously landed in:
https://chromium-review.googlesource.com/c/562018/

but was reverted in:
https://chromium-review.googlesource.com/c/574547/

due to a a Clusterfuzz bug. Turns out that calling UnregisterScrollbar
can erase the references we're holding in scrollbar_ids and
scrollbar_layer_id so we have to refresh those.

Bug: 739738
Change-Id: Ic80018212b1370be6a33a39add19da6848b59fb6
Reviewed-on: https://chromium-review.googlesource.com/578428Reviewed-by: Weiliang Chen <weiliangc@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488030}