[scroll-animations] Handle implicit 'auto' offsets correctly

We currently don't handle that case where start/end offsets are
nullptr correctly. In this case they should behave as 'auto'. Possibly
a regression from the scrollOffsets implementation.

Fixed: 1145916
Bug: 1074052
Change-Id: Ib97e1157b957dc2de4393688967694fc9b0de736
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2520836


Reviewed-by: Olga Gerchikov <gerchiko@microsoft.com>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824632}