postmortem: Remove Context::GLOBAL_OBJECT_INDEX

Context::GLOBAL_OBJECT_INDEX has been replaced by NATIVE_CONTEXT_INDEX in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}

tools/gen-postmortem-metadata.py

@@ -189,7 +189,7 @@ consts_misc = [
 extras_accessors = [
     'JSFunction, context, Context, kContextOffset',
     'Context, closure_index, int, CLOSURE_INDEX',
-    'Context, global_object_index, int, GLOBAL_OBJECT_INDEX',
+    'Context, native_context_index, int, NATIVE_CONTEXT_INDEX',
     'Context, previous_index, int, PREVIOUS_INDEX',
     'Context, min_context_slots, int, MIN_CONTEXT_SLOTS',
     'HeapObject, map, Map, kMapOffset',