Bug 1538015 (part 2) - restrict when incoming synced prefs are applied. r=tcsc,pauljt,Gijs, a=RyanVM
Specifically, a "control pref" for a pref must already exist locally, or
a new preference, `services.sync.prefs.dangerously_allow_arbitrary` must
be set to true.
This also removes a few preferences from the set we sync by default based
due to potential harm which can be caused syncing inappropriate values.
Differential Revision: https://phabricator.services.mozilla.com/D29775
extra : histedit_source : 40454bf4a75a018e29e1a70f930128a43576594d