Revert of Switch V8HiddenValue from hidden values to privates

Revert of Switch V8HiddenValue from hidden values to privates (patchset #7 id:120001 of https://codereview.chromium.org/1416053012/ )

Reason for revert:
This CL broke tests in oilpan builds.

https://storage.googleapis.com/chromium-layout-test-archives/WebKit_Linux_Oilpan_ASAN/5177/layout-test-results/results.html

Original issue's description:
> Switch V8HiddenValue from hidden values to privates
>
> Privates are like ES6 symbols, but are not accesible by JavaScript.
> Hidden values on the other hand will be deleted form the API soon.
>
> BUG=none
> R=haraken@chromium.org
>
> Committed: https://crrev.com/b1802c0fc28635a50e7f55e40c71e69281cc85f3
> Cr-Commit-Position: refs/heads/master@{#360786}

TBR=vogelheim@chromium.org,sigbjornf@opera.com,jochen@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=none

Committed: https://crrev.com/dfb67af2fb65ecb0daec0498140ad6d7a9a0ea59
Cr-Commit-Position: refs/heads/master@{#361016}

[haraken, 2015-11-21 09:01:59]

Created Revert of Switch V8HiddenValue from hidden values to privates

[commit-bot: I haz the power, 2015-11-21 09:02:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1466733006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1466733006/1

[commit-bot: I haz the power, 2015-11-21 09:02:54]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-21 09:03:44]

Patchset 1 (id:??) landed as
https://crrev.com/dfb67af2fb65ecb0daec0498140ad6d7a9a0ea59
Cr-Commit-Position: refs/heads/master@{#361016}

[jochen (gone - plz use gerrit), 2015-11-24 16:52:58]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1473633002/ by jochen@chromium.org.

The reason for reverting is: Underlying oilpan issues were addressed. Esp. the
hidden values are no longer touched in V8AbstractEventListener's dtor.