Add StartCopy and ContinueCopy to SmbProviderClient

This change adds StartCopy and ContinueCopy on SmbProviderClient so that
SmbFileSystem can call these methods on the SmbProvider daemon over
D-Bus. These methods enable chunking units of work of a copy operation
since the SmbProvider daemon is single threaded and we do not want to block
it during long running copies.

Bug: 757625
Change-Id: I3de4df31907c29d872820d13ff5004f82160d663
Reviewed-on: https://chromium-review.googlesource.com/1130821
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575164}
4 files changed