Fix null pointer dereference in AutoscrollController.

A fieldset element creates an anonymous block for its contents (see
LayoutFieldset::InsertedIntoTree), which may be passed as |scrollable|
to AutoscrollController::StartMiddleClickAutoscroll.

It seems we don't need to access scrollable->GetNode() in the first
place.  We can just start the layout tree walk from scrollable itself.

Bug: 1488016
Change-Id: I43da256d48f827c818636848abcd659095a94a7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5076640
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1231408}