This adds a stast phase PARALLEL_MARK which is used for total time on both main
and helper threads. Addtionally, helper threads record PARALLEL_MARK_MARK and
PARALLEL_MARK_WAIT.
The patch also adds three telemetry probes related to parallel marking:
- GC_PARALLEL_MARK_SPEEDUP - Ratio of total helper thread time spent marking to main thread time
- GC_PARALLEL_MARK_UTILIZATION - Ratio of helper thread time spent marking to total helper thread time
- GC_PARALLEL_MARK_INTERRUPTIONS - Number of interruptions/donations per slice
Differential Revision: https://phabricator.services.mozilla.com/D170379