<https://webkit.org/b/123533>

Stop arena-allocating line boxes so we can move forward on improving
render tree memory management. This will also allow more rendering
code to take advantage of malloc optimizations.

This will likely regress performance on some micro-benchmarks, but
it's something we want to do sooner rather than later so we have time
to restabilize it. All improvements to the simple line layout's
coverage will help with recouping whatever is regressed.

BiDi runs are the only remaining user of the arena now.

Reviewed by Antti Koivisto


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@158321 268f45cc-cd09-0410-ab3c-d52691b4dbfc