MozReview-Commit-ID: KmpBdrLmVNK
Also added a hasSetting method to ExtensionSettingsStore to check whether a particular extension has a
setting stored for a particulay key.
This changes the behaviour of ExtensionSettingsStore so that attempting to remove a setting that was not previously set does
not throw an exception. This allows things like privacy.network.webRTCIPHandlingPolicy.clear() to be called without having
previously called privacy.network.webRTCIPHandlingPolicy.set().
MozReview-Commit-ID: FFCOFHk5lhb
