Revert of ServiceWorker: Use scheduler's default task queue for posting tasks on main thread

Revert of ServiceWorker: Use scheduler's default task queue for posting tasks on main thread (patchset #1 id:1 of https://codereview.chromium.org/958523002/)

Reason for revert:
Highly speculative revert after views_unittests broke on win7:

http://build.chromium.org/p/chromium.win/builders/Win7%20Tests%20%28dbg%29%281%29/builds/35660

c:\b\build\slave\win_builder__dbg_\build\src\ui\views\accessibility\native_view_accessibility_win_unittest.cc(190): error: Value of: infobar_accessible.IsSameObject(targets[0])

This patch does change task queuing, which makes it seem slightly more likely to break stuff than the others in the blamelist.

Original issue's description:
> ServiceWorker: Use scheduler's default task queue for posting tasks on main thread
>
> This fixes a task ordering bug between WebMessagePortChannelImpl and
> ServiceWorkerScriptContext. The former posts tasks via the Blink
> scheduler's default task queue, and the latter was using the MessageLoop
> directly. This patch makes tasks from service worker to the main thread
> go through the scheduler.
>
> BUG=460833
> TEST=http/tests/serviceworker/postmessage-msgport-to-client.html
>
> Committed: https://crrev.com/a7fe515e3ca07e260b58ebf460f842601266ed87
> Cr-Commit-Position: refs/heads/master@{#318022}

TBR=kinuko@chromium.org,skyostil@chromium.org,ksakamoto@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=460833
Committed: https://crrev.com/64250e71c15dfe3576d8e4077e4dfb04f9382482
Cr-Commit-Position: refs/heads/master@{#318040}

[phoglund_chromium, 2015-02-25 14:00:34]

Created Revert of ServiceWorker: Use scheduler's default task queue for posting
tasks on main thread

[commit-bot: I haz the power, 2015-02-25 14:01:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/954993002/1

[commit-bot: I haz the power, 2015-02-25 14:01:42]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-25 14:02:12]

Patchset 1 (id:??) landed as
https://crrev.com/64250e71c15dfe3576d8e4077e4dfb04f9382482
Cr-Commit-Position: refs/heads/master@{#318040}

[phoglund_chromium, 2015-02-25 16:27:03]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/957583003/ by phoglund@chromium.org.

The reason for reverting is: Test is still flaky, even after reverting this one.
For instance

http://build.chromium.org/p/chromium.win/builders/Vista%20Tests%20%281%29/bui....