Move oof-descendants to NGPhysicalFragmentBase

As discussed in CR https://codereview.chromium.org/2540653003

As ng_text_fragments can also have oof descendants, oof descendants
should move to NGPhysicalFragmentBase

BUG=635619

Review-Url: https://codereview.chromium.org/2546353002
Cr-Commit-Position: refs/heads/master@{#436477}
7 files changed