[LayoutNG] Force legacy layout for fieldset elements

This is one missed LayoutBlockFlow-derived class that were
not handled yet, either by implementing LayoutNG counterpart
or by forcing legacy.

This isn't a big class, but since most form controls forces
legacy at its cloest BFC ancestor, it's reasonable to handle
when we support form controls.

Bug: 635619
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I9a9bb1788e9fe8367d6ce8d9439aae3f243335de
Reviewed-on: https://chromium-review.googlesource.com/1122028
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571875}
8 files changed