Add new feature flags - mobileid, precompile_asmjs, hardware_512mb_ram (bug 1097428)

https://bugzilla.mozilla.org/show_bug.cgi?id=1097428

Code to use buchets in reviewer tools is removed because it doesn't work properly anyway, as feature detection of privileged APIs doesn't work in the browser.
It should default to false, but I'll make it more explicit.