Fix |LayoutNGBlockFlowMixin::NodeAtPoint()| for block fragmentation

This patch fixes |NodeAtPoint()| so that:
* The |LayoutObject| function does not support block
  fragmented objects.
* Make sure we traverse fragment tree, without going through
  |LayoutObject|, whenever it may have fragmented children.

The current code calls |LayoutBlock::HitTestChildren()| when
|!ChildrenInline()|. It is fixed to traverse fragment tree to
achieve the second point above.

The same philosophy as |Paint| in r824316 <crrev.com/c/2515429>.

Bug: 1061423
Change-Id: Ie153bfab663e9d7353378355b0734e725ed0516d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517215


Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824886}