GitLab

This CL also renames mobile_facebook_login.LoginAccount to
facebook_login.LoginWithMobileSite.

Motivation: later we will add a login method for facebook desktop, hence
generalize this module to support both.

BUG=589726

Review-Url: https://codereview.chromium.org/2166743005
Cr-Commit-Position: refs/heads/master@{#407122}