Fix SelectionController for NG multicol

Following up r824533 <crrev.com/c/2515399> which traverses
fragments for |PositionForPoint|, this patch fixes
|SelectionController| to traverse fragments without going
through |LayoutObject|.

It looks like the code in |SelectionController| has slight
difference from |HitTestResult::GetPosition()| but it does not
look intentional.

The fix is needed to support block fragmentation in
<crrev.com/c/2514048>.

Bug: 829028
Change-Id: I35060e5e2030e3282c60cb11b590105c11f69ca3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522799


Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824887}