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/1173333004.

R=rossberg@chromium.org
LOG=N
BUG=498811
Committed: https://crrev.com/103fcfaa40c23f7c732601d2f4f8be251415a45c
Cr-Commit-Position: refs/heads/master@{#28998}

[wingo, 2015-06-12 12:03:59]

Patchset 1 is the CL most recently applied.  Patchset 2 fixes a nosnap
deserialization test expectation.

[rossberg, 2015-06-12 12:05:29]

lgtm

[wingo, 2015-06-12 12:06:02]

On 2015/06/12 12:03:59, wingo wrote:
> Patchset 1 is the CL most recently applied.  Patchset 2 fixes a nosnap
> deserialization test expectation.

Humm, sometimes rollouts are good.   I realized I forgot to add the regression
test to git!  Patchset 2 adds the test.

[wingo, 2015-06-12 12:07:01]

The CQ bit was checked by wingo@igalia.com

[commit-bot: I haz the power, 2015-06-12 12:07:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1178903003/40001

[commit-bot: I haz the power, 2015-06-12 12:34:19]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-06-12 12:34:36]

Patchset 3 (id:??) landed as
https://crrev.com/103fcfaa40c23f7c732601d2f4f8be251415a45c
Cr-Commit-Position: refs/heads/master@{#28998}