mobile-browser is dead; long live mozilla-central/mobile !

The mobile-browser repo is now a subdirectory of mozilla-central.

• Mozilla-central-based project branches will need to re-sync from m-c to go green.
• Outstanding patches will need to be applied accordingly.
• If there are any outstanding un-ack'ed issues as a result of this merge, please ping me and file a mozilla.org::Release Engineering bug.

On Try, we can still build older, non-mobile-merge builds (e.g., a Fennec 4.0-based patch) by using the mobile-repo and mobile-rev special files as described here.