Add abort functionality to SmbTaskQueue

This change adds Abort functionality to SmbTaskQueue. The unrun tasks for
a given operation can be aborted via SmbTaskQueue::AbortOperation().

Test=unittest

Bug: chromium:757625
Change-Id: I317cc95556e4952a2c7049ad2cdf358d3058147b
Reviewed-on: https://chromium-review.googlesource.com/956260
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542915}
3 files changed