[es6] fix IsConcatSpreadable() algorithm in runtime-array.cc

[es6] fix IsConcatSpreadable() algorithm in runtime-array.cc

The ordering of the "IsArray()" check for IsConcatSpreadable() was incorrect previously --- IsArray() is only used if Get(O, @@isConcatSpreadable) is undefined. Without this fix, it's not possible for Array subclasses to opt out of spreading

22.1.3.1.1 http://www.ecma-international.org/ecma-262/6.0/#sec-isconcatspreadable

BUG=v8:3764
LOG=N
R=arv@chromium.org, dslomov@chromium.org, rossberg@chromium.org

Committed: https://crrev.com/e7d4bf87adcd32da163d8c6a5af82246f4f83edc
Cr-Commit-Position: refs/heads/master@{#29127}

[caitp (gmail), 2015-06-18 15:51:51]

PTAL, quick fixup following allen's comment on
https://esdiscuss.org/topic/isconcatspreadable

[arv (Not doing code reviews), 2015-06-18 17:05:04]

LGTM

Can you change the link to point to the official HTML version instead of Jason's
draft?

[caitp (gmail), 2015-06-18 17:09:15]

On 2015/06/18 17:05:04, arv wrote:
> LGTM
> 
> Can you change the link to point to the official HTML version instead of
Jason's
> draft?

done. Landing asap since I think the bit got flipped for this feature already

[caitp (gmail), 2015-06-18 17:09:21]

The CQ bit was checked by caitpotter88@gmail.com

[commit-bot: I haz the power, 2015-06-18 17:09:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1192153002/1

[caitp (gmail), 2015-06-18 17:10:15]

On 2015/06/18 17:09:15, caitp wrote:
> done. Landing asap since I think the bit got flipped for this feature already

I guess not, huh. But soon

[commit-bot: I haz the power, 2015-06-18 19:47:31]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-18 19:47:42]

Patchset 1 (id:??) landed as
https://crrev.com/e7d4bf87adcd32da163d8c6a5af82246f4f83edc
Cr-Commit-Position: refs/heads/master@{#29127}