[builtins] Fix ToString in Array.prototype.join.

[builtins] Fix ToString in Array.prototype.join.

The internal ConvertToString helper was using the wrong ToPrimitive,
actually the old ES5 like DefaultString, and it also prematurely
optimized for no real benefit.

BUG=v8:4307
LOG=n

Committed: https://crrev.com/1ce206872ff272af1c7da64afda4101b15f1726c
Cr-Commit-Position: refs/heads/master@{#30956}

[Benedikt Meurer, 2015-09-28 04:50:50]

bmeurer@chromium.org changed reviewers:
+ jarin@chromium.org

[Benedikt Meurer, 2015-09-28 04:50:50]

Hey Jaro,

Another obvious one.
Please take a look.

Thanks,
Benedikt

[Jarin, 2015-09-28 05:01:22]

lgtm

[Benedikt Meurer, 2015-09-28 05:02:00]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-09-28 05:02:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1370943002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1370943002/1

[commit-bot: I haz the power, 2015-09-28 05:03:34]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-28 05:03:51]

Patchset 1 (id:??) landed as
https://crrev.com/1ce206872ff272af1c7da64afda4101b15f1726c
Cr-Commit-Position: refs/heads/master@{#30956}