Implement StartCopy and ContinueCopy on SmbFileSystem

This change implements StartCopy and ContinueCopy on SmbFileSystems.
StartCopy will be called by CopyEntry to begin a copy operation. When
the StartCopy handler callback is run, if there is more work to do the
ContinueCopy method is called.

Bug: chromium:757625
Change-Id: I9a8640e6e6b5cd089d65d3d7b668159676121250
Reviewed-on: https://chromium-review.googlesource.com/1132462
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575275}
2 files changed