InlineBox::root() should return a reference.
<https://webkit.org/b/121014>

Reviewed by Antti Koivisto.

There is always a root inline box, so make root() return a reference.
Also make some effort to cache the result in a local at call sites.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@155318 268f45cc-cd09-0410-ab3c-d52691b4dbfc