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It looks like this crashes because of a null outer viewport scroll node.
That should never happen here because we have an inner viewport scroll
node.

There's a few ways this could be going wrong:

- A ScrollNode is marked as the inner scroll node even though it wasn't
  registered as one in viewport_property_ids_
- The ScrollNode doesn't exist in the property tree even though it was
  registered.
- We're getting this bad combination from Blink.

I've turned a few DCHECKs into release CHECKs to narrow down which of
these it might be.

Bug: 1037759
Change-Id: I972582da6e18725557859063da0889d465063c77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1981268
Commit-Queue: David Bokan <bokan@chromium.org>
Auto-Submit: David Bokan <bokan@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728220}