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

Review URL: https://codereview.chromium.org/1439153002

Cr-Commit-Position: refs/heads/master@{#359441}

0 files changed, 0 insertions, 0 deletions