Avoid falling back to legacy if |CanTraversePhysicalFragments|

When painting and hit-testing NG fragments,
|NGBoxFragmentPainter| falls back to |LayoutObject| at the BFC
boundary, because we already have a logic in |LayoutObject|
to switch painters for NG and legacy.

This patch changes to do so only when |!CanTraverse|, because
|LayoutObject| cannot handle block fragmented NG objects.

This patch eliminates |CurrentFragment| from
|LayoutNGBlockFlowMixin::Paint| by making sure it is not block
fragmented.

Also changes |HitTestChildBoxFragment| so that it can handle
atomically-painted objects in NG code path. Due to the change
in |FragmentRequiresLegacyFallback|, such objects go to NG
code path without going through |LayoutObject|.

Bug: 1061423, 829028
Change-Id: Ife31d6dcd209668a489243b1158112dac44fc273
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2515429


Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824316}