Remove 'FloatFlow'

Removes 'FloatFlow' in favor of FloatBlockFlow, which is cointained
inside BlockFlow in a 'has-a' relationship. This avoids a bunch of
duplicated code.

This patch is for:
https://github.com/mozilla/servo/issues/1281
auto merge of #1321 : brunoabinader/servo/remove-floatflow, r=pcwalton

Removes 'FloatFlow' in favor of FloatBlockFlow, which is contained
inside BlockFlow in a 'has-a' relationship. This avoids a bunch of
duplicated code.

This patch is for:
https://github.com/mozilla/servo/issues/1281