Add most recently used mount URL to File Share URL in SMB add dialog

- The text field for File Share URL will now initialize to the most
  recently used share path.

Bug: chromium:887133
Test: end to end
Change-Id: Ic59f7d34332b25c0e28832c6109060b849e838f2
Reviewed-on: https://chromium-review.googlesource.com/c/1300797
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Commit-Queue: jimmy gong <jimmyxgong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604418}
7 files changed