Centralize garbage collection

[marcoscaceres]

Patch https://github.com/web-platform-tests/wpt/commit/b181b1cf64799f29527a4a0a1d5a2bfadb69c5d1 made a few improvements to how tests do garbage collection.

@saschanaz suggested we centralize that somewhere...

@domenic wrote Approving since this is a strict improvement, but some suggestions:

• Copy the more informative comments from streams/
• Use the self.garbageCollect = () => { } format since it makes it clearer that it's adding a global.
• Consolidate these all into /common/gc.js
• Fix the others found in permissions/ and fs/, and the ones found in webusb/ and bluetooth/, and finally the one found in js/builtins.
• Fix various cases that aren't using this, to use this

[gsnedders]

It would be good to reference https://github.com/web-platform-tests/wpt/issues/7899 here, given that's the… original issue about triggering GC?