Andrea Azzarone authored Oct 23, 2018 and Philip Chimento committed Nov 12, 2018

With the switch to mozjs60, the sweeping happens in three phases insted of two:
JSFINALIZE_GROUP_PREPARE, JSFINALIZE_GROUP_START, and JSFINALIZE_GROUP_END.
Update the code to keep track of whether the runtime is currently doing GC
sweeping, and prevent calling JS code at that time.

Fixes: #212