Don't prefill username on "Other user" screen.
Don't prefill username on "Other user" screen.

Old login screen now is only used as "Other user" screen.
Rare case is when you specifically pass --login-screen=[login|oobe|network] and it's not prefilled with the last user. But that's dev test case.

BUG= http://crosbug.com/2853
TEST=Select "Other user" screen from the login screen with avatars. Username field should be empty.

Review URL: http://codereview.chromium.org/2077001

git-svn-id: http://src.chromium.org/svn/trunk/src@47137 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 9eda9e50bf1c67288ecdb7eabb9f98fc341384c8
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser