Inheriting PerThreadAtomCache on CycleCollectedJSContext permits use of
static_cast, avoiding one level of indirection compared to adding a
CycleCollectedJSContext* to PerThreadAtomCache.
PerThreadAtomCache is over 18kB, and so WorkerJSContext and WorkletJSContext
are moved from the stack to the heap.
MozReview-Commit-ID: 6jdJeZcviK4
--HG--
extra : rebase_source : 3c2accb71faf3f017a44c405ae0484e57aaf039c