Use TaskScheduler instead of WorkerPool in user_session_manager.cc.

Use TaskScheduler instead of WorkerPool in user_session_manager.cc.

This CL replaces base::WorkerPool::PostTask() with
base::PostTaskWithTraits() call. The following traits are used:

Priority: BACKGROUND
  User won't notice if this task takes an arbitrarily long time
  to complete.

Shutdown behavior: CONTINUE_ON_SHUTDOWN
  Tasks posted with this mode which have not started executing before
  shutdown is initiated will never run. Tasks with this mode running at
  shutdown will be ignored (the worker will not be joined).

With file IO:
  The task is allowed to perform synchronous IO operations.

No Wait (default):
  The task does not wait on things other than synchronous file IO
  operations (e.g. WaitableEvent, ConditionVariable, join a thread,
  join a process, blocking system call that doesn't involve
  interactions with the file system).

BUG=659191
Committed: https://crrev.com/77c45323d49314e02224ad204cad09b8f052cf8e
Cr-Commit-Position: refs/heads/master@{#436033}

[fdoray, 2016-12-01 21:37:37]

The CQ bit was checked by fdoray@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-12-01 21:38:21]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-12-01 22:04:16]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-12-01 22:04:17]

Dry run: This issue passed the CQ dry run.

[fdoray, 2016-12-02 13:29:55]

fdoray@chromium.org changed reviewers:
+ achuith@chromium.org

[fdoray, 2016-12-02 13:29:55]

Please review this CL. In particular, make sure that the traits mentioned in the
CL description make sense for this task. Thanks!


Note: WorkerPool is being deprecated in favor of TaskScheduler. Read the design
doc
https://docs.google.com/document/d/1S2AAeoo1xa_vsLbDYBsDHCqhrkfiMgoIPlyRi6kxa...


Note: On shutdown, WorkerPool does not wait for tasks to run. To get the same
behavior in TaskScheduler, we used TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN.

[achuithb, 2016-12-02 20:13:47]

lgtm

[fdoray, 2016-12-02 20:30:58]

The CQ bit was checked by fdoray@chromium.org

[commit-bot: I haz the power, 2016-12-02 20:31:31]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-12-02 21:29:46]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1480710658735030, "parent_rev":
"fd04501377fa6db3c12cdc1fbad0da376e97f35b", "commit_rev":
"446cb0575f807e40451262613d749d1314242d4d"}

[commit-bot: I haz the power, 2016-12-02 21:30:15]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-12-02 21:32:45]

Description was changed from

==========
Use TaskScheduler instead of WorkerPool in user_session_manager.cc.

This CL replaces base::WorkerPool::PostTask() with
base::PostTaskWithTraits() call. The following traits are used:

Priority: BACKGROUND
  User won't notice if this task takes an arbitrarily long time
  to complete.

Shutdown behavior: CONTINUE_ON_SHUTDOWN
  Tasks posted with this mode which have not started executing before
  shutdown is initiated will never run. Tasks with this mode running at
  shutdown will be ignored (the worker will not be joined).

With file IO:
  The task is allowed to perform synchronous IO operations.

No Wait (default):
  The task does not wait on things other than synchronous file IO
  operations (e.g. WaitableEvent, ConditionVariable, join a thread,
  join a process, blocking system call that doesn't involve
  interactions with the file system).

BUG=659191
==========

to

==========
Use TaskScheduler instead of WorkerPool in user_session_manager.cc.

This CL replaces base::WorkerPool::PostTask() with
base::PostTaskWithTraits() call. The following traits are used:

Priority: BACKGROUND
  User won't notice if this task takes an arbitrarily long time
  to complete.

Shutdown behavior: CONTINUE_ON_SHUTDOWN
  Tasks posted with this mode which have not started executing before
  shutdown is initiated will never run. Tasks with this mode running at
  shutdown will be ignored (the worker will not be joined).

With file IO:
  The task is allowed to perform synchronous IO operations.

No Wait (default):
  The task does not wait on things other than synchronous file IO
  operations (e.g. WaitableEvent, ConditionVariable, join a thread,
  join a process, blocking system call that doesn't involve
  interactions with the file system).

BUG=659191

Committed: https://crrev.com/77c45323d49314e02224ad204cad09b8f052cf8e
Cr-Commit-Position: refs/heads/master@{#436033}
==========

[commit-bot: I haz the power, 2016-12-02 21:32:46]

Patchset 1 (id:??) landed as
https://crrev.com/77c45323d49314e02224ad204cad09b8f052cf8e
Cr-Commit-Position: refs/heads/master@{#436033}