[Visual Viewport] Fix a new null dereference.

Fixes a null-dereference regression from a recent CL that added
support for scrolling to a position:fixed element when there
a scrollable visual viewport.

Also implements GetSmoothScrollSequencer by getting it from the
main frame if possible.


Change-Id: I75cbb7e25e1a8289e1405f01d2d9ddfafb785361
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1749549
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Auto-Submit: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686414}
2 files changed