Auto merge of #20689 - fabricedesre:mutation-take-records, r=emilio

MutationObserver api: Implement takeRecords() and characterData mutations

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I started by looking at adding support for `takeRecords()` to get fluent.js to work on Servo. To get the wpt tests to pass I ended up implementing character data mutations (most of the plumbing was already there) and this fixed a few more tests.

We are still missing support for `disconnect()`, which I keep for a followup.

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