Revert of Revert of Add a WorkerScheduler and a WebThreadImplForWorker

Revert of Revert of Add a WorkerScheduler and a WebThreadImplForWorker (patchset #1 id:1 of https://codereview.chromium.org/1088933002/)

Reason for revert:
False alarm. The patch was good, the failure was from monday.

Original issue's description:
> 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}

TBR=jochen@chromium.org,rmcilroy@chromium.org,skyostil@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=463143
Committed: https://crrev.com/63c485ecec446c031b98755cb8e3cccecb883448
Cr-Commit-Position: refs/heads/master@{#325043}

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

Created Revert of Revert of Add a WorkerScheduler and a WebThreadImplForWorker

[commit-bot: I haz the power, 2015-04-14 14:37:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1085863002/1

[commit-bot: I haz the power, 2015-04-14 14:38:59]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-14 14:39:51]

Patchset 1 (id:??) landed as
https://crrev.com/63c485ecec446c031b98755cb8e3cccecb883448
Cr-Commit-Position: refs/heads/master@{#325043}