The configuration file will list the themes and their expiry dates

I opted to define the configuration file as an object in a new JSM rather than as standard JSON. We will need to access this configuration data before UI startup, to read the installed theme from it. As a JSM, we can load it with getLazyModuleGetter and access it synchronously. A JSON file requires us to access it with fetch or IOUtils. We'd have to introduce async code to BuiltInThemes.maybeInstallActiveBuiltInTheme, which would be messy seeing as it runs in a sync context. Even though sync fetch or IOUtils would be fast since the file would be in omni.ja, I still don't want to add sync I/O anywhere near startup.

Backed out for causing mochitest failures on BuiltInThemeConfig.jsm

• Backout link
• Push with failures
• Failure Log

Depends on D130306

https://hg.mozilla.org/mozilla-central/rev/7e425f12545a