Acid1 fix (fixed FloatFlow refactory)

While doing the FloatFlow refactory, I forgot to transpose a variable
assignment in assign_widths(). Also did a minor change in
bubble_widths() to avoid creating temporary variables in an iteration
loop.

This patch is for:
https://github.com/mozilla/servo/issues/1335
Merge pull request #1337 from brunoabinader/acid1_fix

Acid1 fix (fixed FloatFlow refactory)