fix gen-postmortem-metadata.py for kInObjectPropertiesOffset

fix gen-postmortem-metadata.py for kInObjectPropertiesOffset

This is causing build breaks for Node.js w/ V8 4.6.

Map::kInObjectPropertiesOffset was been replaced by
kInObjectPropertiesOrConstructorFunctionIndexOffset in
https://codereview.chromium.org/1276533003. This is causing the post-mortem
debug information generation to fail.

R=bmeurer@chromium.org
BUG=

Committed: https://crrev.com/5434d052f138adc0933c20ad24cc3d84555be53d
Cr-Commit-Position: refs/heads/master@{#30536}

[Benedikt Meurer, 2015-09-02 06:42:38]

lgtm

[ofrobots, 2015-09-02 07:21:45]

The CQ bit was checked by ofrobots@google.com

[commit-bot: I haz the power, 2015-09-02 07:21:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1308113007/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1308113007/1

[commit-bot: I haz the power, 2015-09-02 07:45:32]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-02 07:45:48]

Patchset 1 (id:??) landed as
https://crrev.com/5434d052f138adc0933c20ad24cc3d84555be53d
Cr-Commit-Position: refs/heads/master@{#30536}