Revert of Redirect the MessageLoop's task runner to the renderer scheduler

Revert of Redirect the MessageLoop's task runner to the renderer scheduler (patchset #11 id:220001 of https://codereview.chromium.org/987193002/)

Reason for revert:
Speculatively reverting.  It seems to be causing flakiness on the decodeOnOtherThread test: http://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_unit_tests&tests=decodeOnOtherThread

Original issue's description:
> Redirect the MessageLoop's task runner to the renderer scheduler
>
> Configure MessageLoop::task_runner() to post tasks through the scheduler to ensure consistent task ordering.
>
> BUG=465354
>
> Committed: https://crrev.com/f14dd762c01730a81d4a94b64ceba42eaad2d5f2
> Cr-Commit-Position: refs/heads/master@{#338706}

TBR=alexclarke@chromium.org,skyostil@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=465354
Committed: https://crrev.com/70d1a346ddf711d2dc3261b607b375c629888f8a
Cr-Commit-Position: refs/heads/master@{#338736}

[Raymond Toy, 2015-07-14 21:11:08]

Created Revert of Redirect the MessageLoop's task runner to the renderer
scheduler

[commit-bot: I haz the power, 2015-07-14 21:11:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1238483003/1

[commit-bot: I haz the power, 2015-07-14 21:14:12]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-14 21:15:11]

Patchset 1 (id:??) landed as
https://crrev.com/70d1a346ddf711d2dc3261b607b375c629888f8a
Cr-Commit-Position: refs/heads/master@{#338736}

[Noel Gordon, 2015-07-15 03:07:52]

The decoder test recovered after this revert, FTR.