Restructuring of JSObject::preventExtensions.

Restructuring of JSObject::preventExtensions.

Now there are two functions, one corresponding to the spec's
[[PreventExtensions]] and one corresponding to Object.preventExtensions.
They differ in what they return.

This CL is in preparation of implementing Reflect.preventExtensions.

R=rossberg
BUG=

Committed: https://crrev.com/09185f6e2dc0f568d57616dacbc559336defff80
Cr-Commit-Position: refs/heads/master@{#31096}

[rossberg, 2015-10-05 10:31:13]

lgtm

[neis, 2015-10-05 10:52:36]

neis@chromium.org changed reviewers:
+ verwaest@chromium.org

[neis, 2015-10-05 10:54:05]

Toon, can you please check if the UNREACHABLE() in src/objects.c:5976 is okay?
Thanks.

[Toon Verwaest, 2015-10-05 11:04:03]

That unreachable is fine, but I'd still like to keep the return false
afterwards. UNREACHABLE in release is ignored, which isn't great if we would
ever reach it.

[neis, 2015-10-05 11:30:03]

On 2015/10/05 11:04:03, Toon Verwaest wrote:
> That unreachable is fine, but I'd still like to keep the return false
> afterwards. UNREACHABLE in release is ignored, which isn't great if we would
> ever reach it.

OK

[neis, 2015-10-05 11:31:56]

The CQ bit was checked by neis@chromium.org

[neis, 2015-10-05 11:31:56]

The patchset sent to the CQ was uploaded after l-g-t-m from
rossberg@chromium.org
Link to the patchset: https://codereview.chromium.org/1377103005/#ps20001
(title: " ")

[commit-bot: I haz the power, 2015-10-05 11:32:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1377103005/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1377103005/20001

[commit-bot: I haz the power, 2015-10-05 12:08:49]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-05 12:09:04]

Patchset 2 (id:??) landed as
https://crrev.com/09185f6e2dc0f568d57616dacbc559336defff80
Cr-Commit-Position: refs/heads/master@{#31096}