Remove RenderObjectAncestorLineboxDirtySet

setAncestorLineBoxDirty should just use a bitfield. The HashSet shows up as
2-4% in many of my profiles where we insert or remove many nodes. This does
use the last bitfield, but we should seek to refactor RenderObject to free
up more bits.

Review URL: https://codereview.chromium.org/61313005

git-svn-id: svn://svn.chromium.org/blink/trunk@161858 bbb929c8-8fbe-4397-9dbb-9b2b20218538