Revert of Store |detail| as a hidden value of custom event wrappers

Revert of Store |detail| as a hidden value of custom event wrappers (patchset #4 id:60001 of https://codereview.chromium.org/1372513002/ )

Reason for revert:
It caused release blocking bug 537567.

BUG=537567
Original issue's description:
> Store |detail| as a hidden value of custom event wrappers
>
> We cannot hold strong references from Blink to V8. This means
> that we shouldn't use ScriptValue in DOM impl objects.
> To stop using ScriptValue in CustomEvent, store |detail|
> as a hidden value of custom event wrappers and return it when
> the getter of |detail| is called.
>
> BUG=529941
>
> Committed: https://crrev.com/bf91b3e9e1355a0d2dacd2efcdf4d971bd5c1e87
> Cr-Commit-Position: refs/heads/master@{#351242}

TBR=jochen@chromium.org,haraken@chromium.org,bashi@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=529941
Committed: https://crrev.com/1cf9a35fef65ba7124b3a4520c2bf33891e18fc2
Cr-Commit-Position: refs/heads/master@{#351851}

[Xianzhu, 2015-10-01 18:38:47]

Created Revert of Store |detail| as a hidden value of custom event wrappers

[commit-bot: I haz the power, 2015-10-01 18:39:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1383603004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1383603004/1

[commit-bot: I haz the power, 2015-10-01 18:42:24]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-01 18:43:46]

Patchset 1 (id:??) landed as
https://crrev.com/1cf9a35fef65ba7124b3a4520c2bf33891e18fc2
Cr-Commit-Position: refs/heads/master@{#351851}

[haraken, 2015-10-01 23:35:14]

LGTM for the revert, but this CL should be relanded soon (this CL is needed to
fix serious memory leaks).