Replace Au::zero_* with generic methods

[mbrubeck]

Depends on servo/euclid#33.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/1437

This is an external review system which you may optionally use for the code review of your pull request.

In order to help critic track your changes, please do not make in-place history rewrites (e.g. via git rebase -i or git commit --amend) when updating this pull request.

[highfive]

Warning

• These commits modify layout code, but no reftests are modified. Please consider adding a reftest!
• @mbrubeck, please confirm that src/test/html/acid1.html and your favourite wikipedia page still render correctly!

[metajack]

r+

[bors-servo]

saw approval from metajack
at mbrubeck@4821df6

[bors-servo]

merging mbrubeck/servo/zero-geom = 4821df6 into auto

[bors-servo]

mbrubeck/servo/zero-geom = 4821df6 merged ok, testing candidate = 85393c6

[bors-servo]

all tests pass:
success: http://servo-buildbot.pub.build.mozilla.org/builders/linux/builds/1316
success: http://servo-buildbot.pub.build.mozilla.org/builders/mac/builds/1270

[bors-servo]

fast-forwarding master to auto = 85393c6