style: Remove NODE_IS_ANONYMOUS_ROOT.
style: Remove NODE_IS_ANONYMOUS_ROOT.

We no longer have multiple kinds of anonymous subtrees, so we can get back one
node bit.

Differential Revision: https://phabricator.services.mozilla.com/D53344

splav/servo,KiChjang/servo,KiChjang/servo,splav/servo,splav/servo,KiChjang/servo,KiChjang/servo,splav/servo,DominoTree/servo,KiChjang/servo,splav/servo,DominoTree/servo,DominoTree/servo,splav/servo,KiChjang/servo,DominoTree/servo,DominoTree/servo,KiChjang/servo,splav/servo,KiChjang/servo,splav/servo,KiChjang/servo,splav/servo,splav/servo,KiChjang/servo,DominoTree/servo,DominoTree/servo,DominoTree/servo,DominoTree/servo,DominoTree/servo