Simplify file_system_provider::Queue.

Simplify file_system_provider::Queue.

IsAborted() and Remove() were removed, and semantics of Abort() simplified.

TEST=unit_tests: *FileSystemProvider*
BUG=463344
Committed: https://crrev.com/95a1ab38f32e1643b31fe3231f78e5d353529cd4
Cr-Commit-Position: refs/heads/master@{#319041}

[mtomasz, 2015-03-04 06:56:01]

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

[mtomasz, 2015-03-04 06:56:09]

@hirono: PTAL. Thanks.

[hirono, 2015-03-04 09:11:32]

lgtm, thanks!

[mtomasz, 2015-03-04 09:19:24]

Thanks!

[mtomasz, 2015-03-04 09:19:28]

The CQ bit was checked by mtomasz@chromium.org

[commit-bot: I haz the power, 2015-03-04 09:20:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/971303003/1

[commit-bot: I haz the power, 2015-03-04 09:23:39]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-04 09:24:53]

Patchset 1 (id:??) landed as
https://crrev.com/95a1ab38f32e1643b31fe3231f78e5d353529cd4
Cr-Commit-Position: refs/heads/master@{#319041}