Refactor away ScriptThread::alter_resource_utilization().
It's used in two places, one of which already has access to the Window.
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Documents now fully deactivate when a freeze message is sent
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Documents now fully deactivate when a freeze message is sent
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It could be used to have mutable JSVal fields without GC barriers.
With the removal of that trait, MutHeap and MutNullableHeap can respectively
be replaced by MutJS and MutNullableJS.
Report panics using the top-level frame id rather than the pipeline id
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At the moment, we report panics from script and layout using the root pipeline id. Once we are sharing more script threads, there won't be a root pipeline id any more. The plan is only to share script threads in the same tab, so there will be a top-level frame id that all the pipelines have in common. This PR reports panics using that top-level frame id.
This mostly makes a difference to the browser API: rather than targeting a `mozbrowsererror` event at the root iframe of the script thread that panicked, it targets it at the containing mozbrowser iframe.
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