Beta desktop feature: Nearby pages

* Create a 'mobile bridge' that tweaks the desktop UI so mobile code
can run on it and required styles are loaded

* Add hook for GetBetaFeaturePreferences and provide feature

* Add an additional wgMFMode for beta (desktop-beta), update modules so
they can run in this environment

Change-Id: Id81ef4715f545180dbe8f7f2f377e6da29742a05
Merge "Beta desktop feature: Nearby pages"