Fix aborting in throttled file system (FSP).

Fix aborting in throttled file system (FSP).

The previous logic was causing a memory corruption when aborting was not
invoked with the callback, but due to unmounting the file system.

As there are multiple paths for aborting, this CL adds a method IsAborted on
the Queue class so it's possible to check if a task has been aborted before.

TEST=unit_tests: *FileSystemProviderQueue*
BUG=454310
Committed: https://crrev.com/d3126d1fbaec8f9cbf513ac436ae634e2d12b093
Cr-Commit-Position: refs/heads/master@{#318652}

[mtomasz, 2015-03-02 04:24:36]

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

[mtomasz, 2015-03-02 04:24:43]

@hirono: PTAL. Thanks.

[hirono, 2015-03-02 05:14:06]

Just an idea:

1. Queue has the three methods: Enqueue, Abort, Remove
2. If the task is abort before executing, the queue just removes it. Otherwise,
it calls abort callback without removing it from the queue.
3. Once the task is executed, the client code of the queue always has
responsibility to remove it from the queue.

WDYT?

[mtomasz, 2015-03-02 05:30:14]

On 2015/03/02 05:14:06, hirono wrote:
> Just an idea:
> 
> 1. Queue has the three methods: Enqueue, Abort, Remove
> 2. If the task is abort before executing, the queue just removes it.
Otherwise,
> it calls abort callback without removing it from the queue.
> 3. Once the task is executed, the client code of the queue always has
> responsibility to remove it from the queue.
> 
> WDYT?

It's a good idea. Can we however do it separately, as we need to merge this fix
to stable? I'd like to keep the merge as small as possible.

[hirono, 2015-03-02 05:41:36]

Thanks. lgtm for merging it.

[mtomasz, 2015-03-02 05:42:20]

The CQ bit was checked by mtomasz@chromium.org

[mtomasz, 2015-03-02 05:42:35]

On 2015/03/02 05:41:36, hirono wrote:
> Thanks. lgtm for merging it.

Thanks. I'll prepare a proper fix in a separate CL.

[commit-bot: I haz the power, 2015-03-02 05:43:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/968763002/20001

[commit-bot: I haz the power, 2015-03-02 06:13:52]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-02 06:14:34]

Patchset 2 (id:??) landed as
https://crrev.com/d3126d1fbaec8f9cbf513ac436ae634e2d12b093
Cr-Commit-Position: refs/heads/master@{#318652}