Use more internal mutability for DOM objects.

[Ms2ger]

Part of #1854.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/1787

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[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!
• @Ms2ger, please confirm that src/test/html/acid1.html and your favourite wikipedia page still render correctly!

[jdm]

13:51:41     INFO -  task 'ScriptTask' failed at 'RefCell<T> already borrowed', /builds/slave/mac/build/src/compiler/rust/src/libcore/cell.rs:300
13:51:41     INFO -  Pipeline failed in hard-fail mode.  Crashing!
13:51:41     INFO -  test /builds/slave/mac/build/src/test/content/test_window_setInterval.html ... FAILED
13:51:41     INFO -  test /builds/slave/mac/build/src/test/content/test_node_cloneNode.html ... ok
13:51:41     INFO -  failures:
13:51:41     INFO -  ---- /builds/slave/mac/build/src/test/content/test_window_setInterval.html stdout ----
13:51:41     INFO -     task '/builds/slave/mac/build/src/test/content/test_window_setInterval.html' failed at 'Servo exited with non-zero status Ok(exit code: 1)', /builds/slave/mac/build/src/test/harness/contenttest/contenttest.rs:130
13:51:41     INFO -  failures:
13:51:41     INFO -      /builds/slave/mac/build/src/test/content/test_window_setInterval.html