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Currently Guest mode always uses default value of
Preferences.PreferredInputMethod = "" (empty string) that
always leads to "us" keyboard layout (compiled-in default).

Guest sessions should have the following input methods/keyboards readily
available:
 - the VPD value (assuming the latin keyboard representing the underlying
   hardware; should be the default; e.g. French chromebook => azerty)
 - the additional input method/keyboard based on the locale (e.g. Japanese
   locale => should also add the Japanese IME; there are existing rules for
   this)


BUG=329018
TEST=unittest

Review URL: https://codereview.chromium.org/133273032

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247938 0039d316-1c4b-4281-b951-d872f2087c98