Fix a crash during aborting of operations on provided file systems.

Fix a crash during aborting of operations on provided file systems.

In case of an abort, a memory corruption was occuring because of a call to
Queue::Complete() after an operation is aborted, which is invalid according to
both documentation and a DCHECK.

This CL fixes the issue by avoiding calling Queue::Complete() for aborted
tasks.

TEST=Tested manually. Try to copy files from a ZIP archive with thousands of
     files, and abort during the operation.
BUG=454310
Committed: https://crrev.com/33176770fd2364913cb17f251315192b8bf593da
Cr-Commit-Position: refs/heads/master@{#314722}

[mtomasz, 2015-02-04 11:49:29]

mtomasz@chromium.org changed reviewers:
+ hirono@chromium.org

[mtomasz, 2015-02-04 11:49:36]

@hirono: PTAL. Thanks.

[hirono, 2015-02-05 02:16:52]

lgtm!

[mtomasz, 2015-02-05 02:18:26]

The CQ bit was checked by mtomasz@chromium.org

[commit-bot: I haz the power, 2015-02-05 02:18:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/882153006/1

[commit-bot: I haz the power, 2015-02-05 02:22:12]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-05 02:23:20]

Patchset 1 (id:??) landed as
https://crrev.com/33176770fd2364913cb17f251315192b8bf593da
Cr-Commit-Position: refs/heads/master@{#314722}