add SMB Shares settings

This change adds an SMB Shares subpage to the Downloads settings.
If the NativeSmb feature flag is enabled, an SMB Share can be
mounted by providing the url.

Bug: chromium:757625
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ic0b52eeda2247edff8d61b330ecc8a2c755b1bab
Reviewed-on: https://chromium-review.googlesource.com/967160
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546900}
17 files changed