SmbFileSystem: Implement a stub AbortCallback.

- Provides a valid callback so that if it gets called there is no
  crash.
- Currently does not perform an abort since no operations can be
  aborted.
- The file system provider will just ignore any callback that happens
  after it requested above.

Bug: 757625
Change-Id: I5d52eb7117aaa364c78433fba2ab0249e8d8df1f
Reviewed-on: https://chromium-review.googlesource.com/869462
Reviewed-by: May Lippert <maybelle@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529995}
2 files changed