Changes the style of the username label.
Changes the style of the username label.

Background of the label changed to the 35% opaque black color. Label
alignment changed to be left oriented. Also gradient part was added to
the end of the label.

BUG=chromium-os:5756
TEST=Go to login screen (non-first login). Notice changes of the label
appearance.

Review URL: http://codereview.chromium.org/3604008

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@61993 0039d316-1c4b-4281-b951-d872f2087c98

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