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This CL adds AccountPickerDialogFragment that is shown from
SigninFragmentBase and asks the user to select an account from the list.

Bug: 814728
Change-Id: Id82189694c5bf2b9c88ae3434e7a706539a493bc
Reviewed-on: https://chromium-review.googlesource.com/986262
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548153}