[LayoutNG] Consider OOF positioned nodes as IsManagedByLayoutNG.

positioned: fixed NG elements whose containing block was the LayoutView
had issues (the legacy kind) with scrollbars.

We can avoid these issues by letting IsManagedByLayoutNG return true
when OOF. These are basically "managed" by NG as they are laid out
via. the NGOutOfFlowLayoutPart.

The other callers of IsManagedByLayoutNG should also be fine with this
change.
1) https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/frame/local_frame_view.cc;l=1539?q=ismanagedbylayoutng&ss=chromium
   Should be fine, OOF doesn't need to be pre-laid-out.
2) https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/layout/depth_ordered_layout_object_list.cc;l=53?q=ismanagedbylayoutng&ss=chromium
   Should also be fine as OOF?

Bug: 1225548
Change-Id: I42c4551382553ea52edc5f2ada94ce74d570c6bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3033195
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#902402}