The final CC does not happen until after threads are shutdown and so we can't
depend on CC to trigger termination.
This management of the worklet thread by worklet code is an intermediate
situation until worklets run on the threads managed by other objects.
MozReview-Commit-ID: 8hWsdRCppC2
Initial version r=smaug.
Rebased to c616a6fd5e4b by Jan-Ivar Bruaroey <jib@mozilla.com> r=karlt.
Rebased to 83de58ddda20 by Karl Tomlinson <karlt+@karlt.net> r=baku.
MozReview-Commit-ID: G5E5OXydj3a
Initial version r=smaug.
Rebased to c616a6fd5e4b by Jan-Ivar Bruaroey <jib@mozilla.com> r=karlt.
Rebased to 83de58ddda20 by Karl Tomlinson <karlt+@karlt.net> r=baku.
MozReview-Commit-ID: Lo8TWtN8qyz