Revert of Add a WorkerScheduler and a WebThreadImplForWorker

Revert of Add a WorkerScheduler and a WebThreadImplForWorker (patchset #15 id:360001 of https://codereview.chromium.org/1033643004/)

Reason for revert:
Possible cause of webkit_unit_tests failures ImageFrameGeneratorTest.incompleteDecodeBecomesCompleteMultiThreaded and
DeferredImageDecoderTest.decodeOnOtherThread:

http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux/builds/46585

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}

TBR=jochen@chromium.org,rmcilroy@chromium.org,skyostil@chromium.org,alexclarke@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=463143
Committed: https://crrev.com/43983a491ffceda01d98f739886cad5a1eeff846
Cr-Commit-Position: refs/heads/master@{#324861}

[Nate Chapin, 2015-04-13 16:48:00]

Created Revert of Add a WorkerScheduler and a WebThreadImplForWorker

[commit-bot: I haz the power, 2015-04-13 16:49:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1081933002/1

[commit-bot: I haz the power, 2015-04-13 16:51:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-13 16:52:55]

Patchset 1 (id:??) landed as
https://crrev.com/43983a491ffceda01d98f739886cad5a1eeff846
Cr-Commit-Position: refs/heads/master@{#324861}