This enables sharing data with the parent runtime, decreasing memory usage and startup time. Also contains an update to current rust-mozjs, because that's required for this to work.
This adds in preferences for all the SM 39 available options (as
retrieved from Gecko), and uses the same defaults as Gecko. A few
properties are not supported yet, and incremental GC is still always
disabled regardless of the preference setting.
This also adds back in the options that were accidentally deleted when
\#10342 was rebased, which moved things from script_thread.rs to
script_runtime.rs.
This PR mistakenly removed JIT enabling/disabling by preference:
https://github.com/servo/servo/pull/10342/files
Look for `get_pref`.
I'm putting the missing piece of code into the appropriate place in
script_runtime.rs
