Fix #1594 Rename element field of Text and Comment to characterdata

[xiongmao86]

Rename element field of Text and Comment to characterdata.

[highfive]

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

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/708

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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!
• @xiongmao86, please confirm that src/test/html/acid1.html and your favourite wikipedia page still render correctly!

[brson]

Nice fixes! Thanks.

[Ms2ger]

Unfortunately, this also includes #1610, so we can't land as-is. Please ping me when that has landed, so I can review.