layout: Implement reference counting for flows

[pcwalton]

r? @glennw

[highfive]

Warning

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

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/1725

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[glennw]

r+