Revert of Change the update_client task runner behavior to continue on shutdown.

Revert of Change the update_client task runner behavior to continue on shutdown. (patchset #2 id:20001 of https://codereview.chromium.org/1415933011/ )

Reason for revert:
This change is a suspect for causing crbug.com/475872

Original issue's description:
> Change the update_client task runner behavior to continue on shutdown.
>
> Historically, we've had issues with running the BITS COM client on
> threads and interfering with the browser shutdown.
>
> For reason not entirely understood, some out of process COM calls,
> and most common the call to enumerate BITS jobs appear to hang and
> consequently trigger the browser hang shutdown detector.
>
> At first, we had run this code on the FILE thread, then we had moved
> it on blocking pool threads. However, the net effect is that the
> hang moved as well.
>
> This change avoids blocking the shutdown by allowing the code to
> run after the browser shutdown until the OS terminates the thread as
> part of the process exit.
>
> While it is somehow difficult to reason about the correctness of
> the update_client code, this change is reasonably safe to make
> due to aspects of refcounting and containment of the update_client
> such as not accessing browser global state and refcounting the
> objects that are thread aware.
>
> BUG=552028
>
> Committed: https://crrev.com/5081941b99d37952e1c60c9cb24257369d9c6416
> Cr-Commit-Position: refs/heads/master@{#359138}

TBR=waffles@chromium.org,asargent@chromium.org,rockot@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=552028
Committed: https://crrev.com/f4106b804bd4a023c044de4a6b2c4f5e564b0954
Cr-Commit-Position: refs/heads/master@{#359441}

[Sorin Jianu, 2015-11-12 23:52:38]

Created Revert of Change the update_client task runner behavior to continue on
shutdown.

[commit-bot: I haz the power, 2015-11-12 23:53:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1439153002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1439153002/1

[commit-bot: I haz the power, 2015-11-12 23:55:13]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-12 23:56:10]

Patchset 1 (id:??) landed as
https://crrev.com/f4106b804bd4a023c044de4a6b2c4f5e564b0954
Cr-Commit-Position: refs/heads/master@{#359441}

[Sorin Jianu, 2015-11-13 18:17:45]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1440393002/ by sorin@chromium.org.

The reason for reverting is: The original change has been ruled out as the cause
for https://code.google.com/p/chromium/issues/detail?id=475872

Therefore, this is the revert of the revert for
https://codereview.chromium.org/1415933011/

When this patch lands, the state of the code will be that we would have the
intended BITS threading changes in, but BITS itself will still be disabled due
to https://codereview.chromium.org/1441223002/

Another change to re-enable BITS will land shortly..