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Map::kInObjectPropertiesOffset was been replaced by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=

Review URL: https://codereview.chromium.org/1308113007

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#30536}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh