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- Marks username and password as optional in the add share dialog.

Bug: chromium:891020
Change-Id: Ie3ea8c2a021dd6be42a129865aac30fe00f67dcb
Reviewed-on: https://chromium-review.googlesource.com/1255900Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595645}