Feature Flag + Mobile Sync

[danfinlay]

Replaces #5935.

Hides that feature (which doesn't make sense until we have mobile) behind a hidden feature flag.

Adds the ability to set feature flags from the background console using this method:

/*
* @method
* Sets a feature flag to the `enabled` value.
* @param { String } featureName - The unique name for the feature.
* @param { Boolean } enabled - Whether the feature should be enabled.
*/
function setPreference (featureName, enabled) { /* implementation */ }

You can try it out by entering setPreference('mobileSync', true) in the background console, which will add a new button to the bottom of the settings view.

[brunobar79]

Adding this comments so I can address before we decide to merge this in

[metamaskbot]

Builds ready [51b826f]: mascara, chrome, firefox, edge, opera

[metamaskbot]

Builds ready [42cb688]: mascara, chrome, firefox, edge, opera

[metamaskbot]

Builds ready [0f55566]: mascara, chrome, firefox, edge, opera

I can't review myself lol

[danfinlay]

There's an important item to note with this feature: Since it hangs a method on the global namespace, preferences managed are highly exposed to all dependencies, so we should make sure that none of the preferences have any potential for abuse by a malicious dependency.

[danfinlay]

Ok I've pushed up some comments to help encourage correct future use of the feature flags hash. I'm ok with this being merged if someone approves it now.

[metamaskbot]

Builds ready [457c831]: mascara, chrome, firefox, edge, opera

[metamaskbot]

Builds ready [99a3c5c]: mascara, chrome, firefox, edge, opera

[whymarrh]

Two small things:

1. The noted regression (?) in the PreferencesController
2. The changes to the package-lock.json file should probably be dropped and re-added, I'm seeing this PR downgrade the version of json-rpc-engine we're pinned to from 3.8.0 to 3.7.3 which doesn't seem correct.

[whymarrh]

We should drop this addition before merging as this seems to be a regression

[metamaskbot]

Builds ready [c2c56f0]: mascara, chrome, firefox, edge, opera

[whymarrh]

Looks fine to me

[guix77]

@danfinlay on MM 6.5.3 (FF + Chrome) under Settings > Advanced, the mobile sync is not hidden any more. Does not help with confusion about the "release" of the mobile app medias talked about and malware fake apps :)