layout: Stop adding damage when removing whitespace.

Avoids total reflow of the entire document on the maze solver.
auto merge of #3841 : pcwalton/servo/removing-whitespace-damage, r=cgaebel

Avoids total reflow of the entire document on the maze solver.

I have tested Wikipedia reflow and it still works.

r? @cgaebel
layout: Promote absolute positioning, floatedness, and clearance into
flags to avoid virtual calls.

These were showing up really high in the maze solver profile.