Implement missing functionality of AudioBufferSourceNode
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This is a tiny PR to support [Media #293](https://github.com/servo/media/pull/293). Last time I ran WPT there was only one failing AudioBufferSourceNode test (which I will debug later this week). Once I rebased, however, I started getting unrelated servo-media build failures:
```
error[E0599]: no method named `shutdown_audio_context` found for type `std::sync::Arc<servo_media::ServoMedia>` in the current scope
--> components/script/dom/baseaudiocontext.rs:566:14
error[E0599]: no method named `shutdown_player` found for type `std::sync::Arc<servo_media::ServoMedia>` in the current scope
--> components/script/dom/htmlmediaelement.rs:1866:18
```
I wanted to get this PR up so https://github.com/servo/media/pull/293 can be tested. When the unrelated build failures are fixed, I will run `./mach update-wpt` and update this PR.
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Finish the JSON clone algorithm for WebDriver
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This makes some tests regarding clearing rects pass. Otherwise the rect
wouldn't get cleared properly and it'd keep its original color but get
an alpha of 0
I tried to mimic the azure implementation but raqote doesn't have a
concept of horizontal or vertical repeat so this might be
wrong/incomplete but this is the last function needed to finally have
some tests pass
Use ipc router in webdriver
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Fix for https://github.com/servo/servo/issues/23905#issuecomment-522588119
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Support default toJSON in WebIDL
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Ported related lines from gecko.
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