Revert of base: Make it possible to replace the MessageLoop's task runner

Revert of base: Make it possible to replace the MessageLoop's task runner (patchset #7 id:120001 of https://codereview.chromium.org/998063002/)

Reason for revert:
Reverting this CL because of https://crbug.com/508789
Please fix and test using the linux_chromium_tsan_rel_ng trybot.

Original issue's description:
> base: Make it possible to replace the MessageLoop's task runner
>
> This patch makes it possible to customize the task posting behavior
> of a MessageLoop. More specifically, a client can change the value
> returned by MessageLoop::task_runner() as well as
> ThreadTaskRunnerHandle::Get() on the target thread. The original task
> runner can still be used to post tasks to be run on the message loop.
>
> The Blink/renderer scheduler will use this functionality to manage task
> posting on the renderer main thread. This is needed to ensure consistent
> ordering of tasks posted through the MessageLoop w.r.t. tasks posted to
> the scheduler.
>
> Design doc: https://docs.google.com/a/chromium.org/document/d/1qxdh2I61_aB_Uzh1QgNqvdWFBCL_E65G2smoSySw7KU/edit#
>
> Alex Clarke <alexclarke@chromium.org>; also contributed to this patch (https://codereview.chromium.org/1206893003/).
>
> BUG=465354
>
> Committed: https://crrev.com/698e77997894bf823c45ad1d6e0764fa93e2f3c1
> Cr-Commit-Position: refs/heads/master@{#337799}

TBR=jam@chromium.org,danakj@chromium.org,alexclarke@google.com,kinuko@chromium.org,skyostil@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=465354
Committed: https://crrev.com/3dd3cd58b7bcd8efe40552a1bb9a9cc2a3679405
Cr-Commit-Position: refs/heads/master@{#338484}

[Alexander Potapenko, 2015-07-13 09:10:22]

Created Revert of base: Make it possible to replace the MessageLoop's task
runner

[commit-bot: I haz the power, 2015-07-13 09:10:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1223193004/1

[commit-bot: I haz the power, 2015-07-13 09:11:16]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-13 09:12:22]

Patchset 1 (id:??) landed as
https://crrev.com/3dd3cd58b7bcd8efe40552a1bb9a9cc2a3679405
Cr-Commit-Position: refs/heads/master@{#338484}