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

Revert of Revert of ServiceWorker: Use scheduler's default task queue for posting tasks on main thread (patchset #1 id:1 of https://codereview.chromium.org/954993002/)

Reason for revert:
Test is still flaky, even after reverting this one. For instance

http://build.chromium.org/p/chromium.win/builders/Vista%20Tests%20%281%29/builds/53768

Original issue's description:
> 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}

TBR=kinuko@chromium.org,skyostil@chromium.org,ksakamoto@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=460833
Committed: https://crrev.com/bdf542901eb439619dfc221fda500d4876fad3f4
Cr-Commit-Position: refs/heads/master@{#318058}

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

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