Assorted SpiderMonkey GC-related fixes

[jdm]

These changes combined allow running a debug-mozjs Servo build with JS_GCZEAL=2,20 (ie. perform a GC every 20 allocations).

r? @Ms2ger

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/5096

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[jdm]

@glennw These changes made the jQuery test suite hum along without incident for me.

[glennw]

@jdm Fantastic! Thank you for looking into it! :)

[michaelwu]

If #6150 can also run the jQuery test suite, I would prefer avoiding these changes. RootedVecs are fixed a bit differently and the issue with the global is part of a deeper issue that affects all dom objects being created/wrapped. The dom object being wrapped needs to be rooted while the wrapper is being setup because the dom object might point to other dom objects that aren't rooted.

[Ms2ger]

So when is Void::reflector called?

[nox]

@Ms2ger AFAIK, in the loop over dom_collections in RootedCollectionSet::trace().

[Ms2ger]

If we're going to do this, I'd like to split up Void into one for storing in RootedVec and one for JS.

[metajack]

After discussion with @jdm he split off #6247 for the stuff that makes jquery test suite work. This will remain open for now until after the SM upgrade lands and we can look at what's still useful.

[Ms2ger]

Please rebase on master.

[metajack]

JS upgrade landed. Is any of this still useful?

[jdm]

I suspect no.