Enable webxr by default
This changes two things:
* enable WebXR by default, since it's not exactly an experimental technology in Servo anymore
* enable the glwindow webxr backend by default so it's easier to test WebXR on desktop nightlies
Since the glwindow pref is only checked in the glutin desktop port, enabling it for other ports does not hurt us at all and it removes a papercut for testing WebXR content on desktop.
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Async wasm compilation event loop integration
The PR contains changes related to binding the runnable dispatching in script_runtime and is part of the Asynchronous WebAssembly Compilation fix. This is the first step in the subsequent steps mentioned in the [wiki](https://github.com/servo/servo/wiki/Asynchronous-WebAssembly-compilation-project).
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Fix loophole in messageport transfer
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layout: Ensure abs-pos elements establish block formatting contexts
Updating #16310
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Reduce unnecesarily large borrow in script thread
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- [x] These changes fix#24730
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Add start_time to resource timing.
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`start_time` property added to `ResourceFetchTiming`, which enables the setting of `start_time` in the `PerformanceEntry` member of `PerformanceResourceTiming`.
Following the specification at https://w3c.github.io/resource-timing/#dfn-starttime, `start_time` is set to the value of `redirect_start` if redirection occurs and the timing allow check passes. Otherwise it has the same value as `fetch_start`.
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- [x] These changes fix#21254
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Continue message port
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Fixes#7457.
Fixes#12715.
Fixes#12717.
Fixes#16095.
Fixes#18969.
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Cache crossorigin test
This adds a test for #24356 to ensure that we don't regress our cache behaviour.
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Temporary implementation of Canvas.MeasureText
@jdm: that will help with the babylonjs demos. Another approach is to implement that in JS and add it to the demos.
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