Rename starts_*_flow to is_*_flow

[lpy]

see #1692

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/789

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Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @jdm (or someone else) soon.

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Warning

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