Fix bug in ExtractMountNameAndFullPath to set volume root as "/"

Bug found coding/testing
https://chromium-review.googlesource.com/c/chromium/src/+/1350375

Bug: 878324
Change-Id: I800bf263f17370d43c7b8413cd6af9cd819bfd0c
Reviewed-on: https://chromium-review.googlesource.com/c/1350421
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610972}
2 files changed