Revert of Remove the concept of a cleanup task

Revert of Remove the concept of a cleanup task (patchset #10 id:180001 of https://codereview.chromium.org/1111693003/)

Reason for revert:
This is also causing flaky failures on the regular Linux ASAN bot:

https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20ASAN/builds/18485

is the first of multiple failures. I am able to reproduce the failure 100% of the time locally on my linux desktop with this patch; reverting the patch makes the test pass.

Sorry!

Original issue's description:
> Remove the concept of a cleanup task
> 
> The only cleanup task in Blink which is not internal to a worker thread is
> the task to close a WebSQL database. Because 1) only worker threads
> support the concept of a cleanup task, and 2) WebSQL isn't supported on
> worker threads anymore after https://codereview.chromium.org/561093003/,
> we can safely remove the logic to handle cleanup tasks on worker
> threads.
> 
> BUG=463143
> TBR=jochen@chromium.org
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=195191

TBR=kinuko@chromium.org,haraken@chromium.org,alexclarke@chromium.org,jochen@chromium.org,sigbjornf@opera.com,skyostil@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=463143
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=195278

[Dirk Pranke, 2015-05-12 22:48:46]

Created Revert of Remove the concept of a cleanup task

[commit-bot: I haz the power, 2015-05-12 22:49:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1134933003/1

[commit-bot: I haz the power, 2015-05-12 22:49:33]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=195278

[haraken, 2015-05-12 23:00:11]

LGTM