This patch is mainly to make IdleTaskRunner reusable by nsHtml5TreeOpExecutor.
The only necessary work to that purpose is to remove the dependency of
sShuttingDown, which was a static variable in nsJSEnvironment.cpp.
The idea is to have a "MayStopProcessing" as a callback for the consumer to
return sShuttingDown.
In addition to sShuttingDown, we use std::function<bool()> as the runner
main callback type.
MozReview-Commit-ID: FT2X1unSvPS
503780c382