Implement non-visible pipeline and iframe visibility methods
This addresses #9566 and a good part of #9751, specifically:
* Pipeline has a notion of visibility
* IFrame setVisible/getVisible interface with IFrame's pipeline visibility
* IFrame mozbrowservisibilitychange responds to changes in visibility
* Pipeline visibility is used to limit animations (requestAnimationFrame does not tick animations when hidden) and to increase timer intervals (currently set to a minimum of 1 second while hidden)
Absent for now are any changes to the Document API and general implementation of the Page Visibility API, since the more interesting parts require knowledge of whether the user agent is minimized, OS screen locked, etc.
cc @paulrouget @jdm
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It's not clear to me if this is something we should expect to happen, but it
does indeed happen, so we should disable the assertion while we investigate.
Fixes#9984.
The code was split into the following two abstractions.
- OneshotTimers can be used to schedule arbitrary oneshot timers, such
as XHR-Timeouts.
- JsTimers (`setTimeout` and `setInterval`) which use OneshotTimers to
schedule individual callbacks.
With this change the implementation (of JsTimers in particular) is in
much closer alignment with the specification.
Functions returning `Root<T>` are prefixed by "root_" and the ones returning
`*const T` by "native_".
Functions taking `*mut JSObject` are now suffixed by "_from_object" and the ones
taking `&T` by "_from_reflector".
Fixed panic in ActiveTimers.set_timeout_or_interval.
`ActiveTimers.set_timeout_or_interval` asserts that the pipeline is not currently frozen. Apparently that is too strict. When pending network requests complete after a pipeline is frozen, scripts may be executed and a timer scheduled.
With these changes scheduling a timer while the pipeline is frozen behaves as if the timer was scheduled at the time the pipeline was frozen.
To reproduce the panic
1. `./mach run -r http://google.com`,
2. immediately click on any link and
3. wait for the panic.
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This used to conflict with the util crate from the standard library, which
has long since been removed.
The import in layout has not been changed because of a conflict with the
util mod there.
