Revert of Revert of Enable Service Workers, including the onfetch event, by default. (patchset #2 id:70001 of https://codereview.chromium.org/678843002/)

Reason for revert:
It is ok to enable SW.
We will fix this issue before launch.

Original issue's description:
> Revert of Enable Service Workers, including the onfetch event, by default. (patchset #1 id:1 of https://codereview.chromium.org/680473002/)
> 
> Reason for revert:
> Directory restriction of ServiceWorker sope patch (https://codereview.chromium.org/672383003/) is reverted.
> 
> We should not enable ServiceWorker without this restriction for security reason.
> 
> I will revert this patch after these patches will land.
> [1/3 blink] https://codereview.chromium.org/674133004/
>  Update LayoutTests to be subject to the directory restrictions.
> [2/3 chromium] https://codereview.chromium.org/674133004/
>  Introduce the directory restriction in Chromium side.
> [3/3 blink] https://codereview.chromium.org/681483004/
>  Introduce the directory restriction in Blink side and update LayoutTest.
> 
> 
> Original issue's description:
> > Enable Service Workers, including the onfetch event, by default.
> > 
> > BUG=364627
> > TEST=virtual/stable/webexposed
> > 
> > Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=184340
> 
> TBR=jochen@chromium.org,tkent@chromium.org,falken@chromium.org,dominicc@chromium.org
> NOTREECHECKS=true
> NOTRY=true
> BUG=364627
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=184400

TBR=jochen@chromium.org,tkent@chromium.org,falken@chromium.org,dominicc@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=364627

Review URL: https://codereview.chromium.org/680643004

git-svn-id: svn://svn.chromium.org/blink/trunk@184410 bbb929c8-8fbe-4397-9dbb-9b2b20218538