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This CL adds language and keyboard pickers to public session pods. The
CL focuses on the necessary changes to the login UI. Follow-up CLs will:
- wire up the pickers so that they actually influence the language and
  keyboard layout in the public session, and
- customize the contents of the language picker based on policy
  (languages recommended by policy go first; the top recommended language
  is pre-selected)

BUG=214904,241790
TEST=Manual, including RTL

Review URL: https://codereview.chromium.org/398543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284983 0039d316-1c4b-4281-b951-d872f2087c98