Revert of Add a WorkerScheduler and a WebThreadImplForWorker

Revert of Add a WorkerScheduler and a WebThreadImplForWorker (patchset #16 id:380001 of https://codereview.chromium.org/1033643004/)

Reason for revert:
Looks like this broke ImageFrameGeneratorTest.incompleteDecodeBecomesCompleteMultiThreaded

Original issue's description:
> Add a WorkerScheduler and a WebThreadImplForWorker
>
> Adds a simple WorkerScheduler which supports Default and Idle Tasks (via
> Long Idle Periods). A follow up Blink patch will refactor
> WorkerThread.cpp to use these Idle Tasks.
>
> A WorkerSceduler is a pre-requisite for refactoring away the Blink
> Timer Heap as described in:
> https://docs.google.com/document/d/163ow-1wjd6L0rAN3V_U6t12eqVkq4mXDDjVaA4OuvCA/edit?usp=sharing
>
> BUG=463143
>
> Committed: https://crrev.com/510ed9c42029e1b2924ac05082c958591f029624
> Cr-Commit-Position: refs/heads/master@{#324847}
>
> Committed: https://crrev.com/b37d4c285dc2f505b006187d2651f6db96d9a42c
> Cr-Commit-Position: refs/heads/master@{#325026}

TBR=jochen@chromium.org,rmcilroy@chromium.org,skyostil@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=463143
Committed: https://crrev.com/ebe9b674fb28f45d78dda654ddb85c7cdb127953
Cr-Commit-Position: refs/heads/master@{#325037}

[alex clarke (OOO till 29th), 2015-04-14 13:52:22]

Created Revert of Add a WorkerScheduler and a WebThreadImplForWorker

[commit-bot: I haz the power, 2015-04-14 13:52:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1088933002/1

[alex clarke (OOO till 29th), 2015-04-14 13:56:03]

The CQ bit was checked by alexclarke@chromium.org

[commit-bot: I haz the power, 2015-04-14 13:56:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1088933002/1

[commit-bot: I haz the power, 2015-04-14 13:57:57]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-14 13:59:26]

Patchset 1 (id:??) landed as
https://crrev.com/ebe9b674fb28f45d78dda654ddb85c7cdb127953
Cr-Commit-Position: refs/heads/master@{#325037}

[alex clarke (OOO till 29th), 2015-04-14 14:37:31]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1085863002/ by alexclarke@chromium.org.

The reason for reverting is: False alarm. The patch was good, the failure was
from monday..