[LayoutNG] Move |PaintFragment()| to |LayoutObject|

This function was in |LayoutBlockFlow|, and it is still
guaranteed that only |LayoutNGMixin| can return non-null
values.

Moving to |LayoutObject| can save checking subclasses in many
cases, which involves one virtual call.

Bug: 636993
Change-Id: Icad922c019e4b09bd36af5928d2f09710c57cf30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1512978
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639372}
6 files changed