Context::GLOBAL_OBJECT_INDEX has been replaced by NATIVE_CONTEXT_INDEX in
https://codereview.chromium.org/1480003002. Update the postmortem data generator
to reflect this change.

R=bmeurer@chromium.org,yangguo@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1542753002

Cr-Commit-Position: refs/heads/master@{#32993}