Add script context with context-allocated "const this"

Add script context with context-allocated "const this"

This is a reapplication of https://codereview.chromium.org/1179893002/.

R=rossberg@chromium.org,mstarzinger@chromium.org
LOG=N
BUG=498811
Committed: https://crrev.com/cfc764f4050edc74aef92daa4c39ccc113893968
Cr-Commit-Position: refs/heads/master@{#28994}

[rossberg, 2015-06-12 10:40:16]

What has changed?

[wingo, 2015-06-12 10:46:57]

On 2015/06/12 10:40:16, rossberg wrote:
> What has changed?

Patchset 1 is the original patchset.  Patchset 2 fixes the gcmole issue, I
think.

[Michael Starzinger, 2015-06-12 10:48:25]

LGTM on patch set #2.

[wingo, 2015-06-12 10:49:11]

The CQ bit was checked by wingo@igalia.com

[commit-bot: I haz the power, 2015-06-12 10:49:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1173333004/20001

[commit-bot: I haz the power, 2015-06-12 11:11:18]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-12 11:11:38]

Patchset 2 (id:??) landed as
https://crrev.com/cfc764f4050edc74aef92daa4c39ccc113893968
Cr-Commit-Position: refs/heads/master@{#28994}

[wingo, 2015-06-12 11:52:42]

A revert of this CL (patchset #2 id:20001) has been created in
https://codereview.chromium.org/1177903004/ by wingo@igalia.com.

The reason for reverting is: nosnap failure.