Add call to file_system_provider::Service::UnmountFileSystem().

This change adds a call to file_system_provider::Service::UnmountFileSystem()
to SmbFileSystem::HandleRequestUnmountCallback() via SmbService, fixing a bug
where smb mounts were not actually removed from VolumeManager when
unmounting via Files App.

Bug: 757625
Test: Verified manually on eve.
Change-Id: Ib38fd944a04e589c33670b084fd052a74bfeb9b1
Reviewed-on: https://chromium-review.googlesource.com/850819
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528840}
6 files changed