[stubs] Use a single slot for context globals.

[stubs] Use a single slot for context globals.

Don't use different read/write slots for context globals, but
let them share the same slot, which reduces the number of
initial misses, and also saves some memory for large scripts.

R=yangguo@chromium.org

Committed: https://crrev.com/4eeafb8802aaf32dac03b91ec61e166f45b76f81
Cr-Commit-Position: refs/heads/master@{#29889}

[Benedikt Meurer, 2015-07-28 07:31:16]

Hey Yang,

This is a followup optimization for context globals, which is now possible since
we handle readonly properties correctly (in the fast case for stores).
Please take a look.

Thanks,
Benedikt

Igor: FYI

[Yang, 2015-07-28 08:11:54]

On 2015/07/28 07:31:16, Benedikt Meurer wrote:
> Hey Yang,
> 
> This is a followup optimization for context globals, which is now possible
since
> we handle readonly properties correctly (in the fast case for stores).
> Please take a look.
> 
> Thanks,
> Benedikt
> 
> Igor: FYI

LGTM. This is great!

[Benedikt Meurer, 2015-07-28 08:12:06]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-07-28 08:12:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1258213002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1258213002/1

[commit-bot: I haz the power, 2015-07-28 08:18:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-28 08:18:42]

Patchset 1 (id:??) landed as
https://crrev.com/4eeafb8802aaf32dac03b91ec61e166f45b76f81
Cr-Commit-Position: refs/heads/master@{#29889}