[turbofan] Fix context chain extension for top-level code.

[turbofan] Fix context chain extension for top-level code.

For top-level code the closure passed into context allocation methods
needs to be replaced with a sentinel to canonicalize is to the empty
function object.

R=bmeurer@chromium.org
TEST=mjsunit/regress/regress-4169
BUG=v8:4169
LOG=N

Committed: https://crrev.com/eb0593e1376f84bc058406ee0ca387d98858ba9d
Cr-Commit-Position: refs/heads/master@{#28888}

[Benedikt Meurer, 2015-06-10 04:44:04]

LGTM.

[Benedikt Meurer, 2015-06-10 06:01:41]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-06-10 06:01:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1172013002/1

[commit-bot: I haz the power, 2015-06-10 06:03:08]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-10 06:03:23]

Patchset 1 (id:??) landed as
https://crrev.com/eb0593e1376f84bc058406ee0ca387d98858ba9d
Cr-Commit-Position: refs/heads/master@{#28888}