Add SmbUrl class

Adds SmbUrl class which is used to represent Smb URLs. Includes unit
tests.

Bug: 757625
Change-Id: I2105f08a0df55dbaa0d50b1637d419a6cf5335f7
Reviewed-on: https://chromium-review.googlesource.com/1048745
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Allen Vicencio <allenvic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557223}