[runtime] Make elements backing store for arguments non-holey.

[runtime] Make elements backing store for arguments non-holey.

Change sloppy and strict arguments objects to FAST_ELEMENTS for their
backing stores, which will allow us to optimize them easier. Also fixes
a bug in the elements accessors that was triggered by this change.

R=verwaest@chromium.org

Committed: https://crrev.com/d49bb69a5e327298157079adc8a301e9829fb7a5
Cr-Commit-Position: refs/heads/master@{#31942}

[Benedikt Meurer, 2015-11-11 10:00:17]

Hey Toon,

This is the change we were working.
Please take a look.

Thanks,
Benedikt

[Toon Verwaest, 2015-11-11 11:49:45]

lgtm

[Benedikt Meurer, 2015-11-11 11:53:56]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-11-11 11:54:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1433133003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1433133003/20001

[commit-bot: I haz the power, 2015-11-11 12:08:30]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-11-11 12:08:55]

Patchset 2 (id:??) landed as
https://crrev.com/d49bb69a5e327298157079adc8a301e9829fb7a5
Cr-Commit-Position: refs/heads/master@{#31942}