cros: Break LoginUserMenuView into a separate class

Bug: 912658
Change-Id: Ib56d677c3ffe5487b7861d0e4a032babd0c4d44d
Reviewed-on: https://chromium-review.googlesource.com/c/1401609
Commit-Queue: Quan Nguyen <qnnguyen@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622146}
14 files changed