Convert all DOM types from @mut to JSManaged.

[jdm]

No description provided.

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

[jdm]

Still to do:

• Rebase
• Address bikesheds - JSManaged -> JS, from/to -> up/down, value/mut_value -> get/get_mut

[pcwalton]

This is awesome. 🤘

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/684

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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.

[jdm]

@kmcallister If you can sign off on my changes relating to the UnsafeFlow/PaddedUnsafeFlow stuff, this is basically ready to land after a final rebase.

[jdm]

r=Ms2ger,kmcallister

[bors-servo]

saw approval from Ms2ger
at jdm@6253254

[bors-servo]

merging jdm/servo/jsmanaged = 6253254 into auto

[bors-servo]

jdm/servo/jsmanaged = 6253254 merged ok, testing candidate = ffcf3b2

[bors-servo]

all tests pass:
success: http://servo-buildbot.pub.build.mozilla.org/builders/linux/builds/967
success: http://servo-buildbot.pub.build.mozilla.org/builders/mac/builds/922

[bors-servo]

fast-forwarding master to auto = ffcf3b2