Revert of Use TaskScheduler instead of WorkerPool in nss_cert_database.cc. (patchset #4 id:60001 of https://codereview.chromium.org/2603173002/ )

Reason for revert:
Using TaskScheduler with NSS is not safe until TaskScheduler supports dynamically growing its thread pool.

Original issue's description:
> Use TaskScheduler instead of WorkerPool in nss_cert_database.cc.
>
> Also, remove NSSCertDatabase::SetSlowTaskRunnerForTest(). This method
> was used to run slow NSSCertDatabase tasks synchronously in tests. This
> is no longer useful since tasks posted through the post_task.h API within
> the scope of a ScopedTaskScheduler run synchronously.
>
> The following traits are used to post tasks to TaskScheduler:
>
> Priority: Inherited (default)
>   The priority is inherited from the calling context (i.e. TaskTraits
>   are initialized with the priority of the current task).
>
> 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).
>
>   Note: Tasks that were previously posted to base::WorkerPool should
>   use this shutdown behavior because this is how base::WorkerPool
>   handles all its tasks.
>
> May Block:
>   Tasks posted with MayBlock() may block. This includes but is not
>   limited to tasks that wait on synchronous file I/O operations:
>   read or write a file from disk, interact with a pipe or a socket,
>   rename or delete a file, enumerate files in a directory, etc. This
>   trait isn't required for the mere use of locks.
>
> BUG=659191
>
> Review-Url: https://codereview.chromium.org/2603173002
> Cr-Commit-Position: refs/heads/master@{#448059}
> Committed: https://chromium.googlesource.com/chromium/src/+/982a31fe2c2dc31014dfaf373c6e5da6f094dcc4

TBR=rsleevi@chromium.org,stevenjb@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=659191

Review-Url: https://codereview.chromium.org/2722733002
Cr-Commit-Position: refs/heads/master@{#453475}