Return refererence from InlineBox::root()

Since there is always root inline box, InlineBox::root() should return
reference.

BUG=

Review URL: https://codereview.chromium.org/182413005

git-svn-id: svn://svn.chromium.org/blink/trunk@168391 bbb929c8-8fbe-4397-9dbb-9b2b20218538