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Map::kInObjectPropertiesOffset was been replaced by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: refs/heads/master@{#30536}

tools/gen-postmortem-metadata.py

@@ -142,7 +142,7 @@ extras_accessors = [
     'JSObject, elements, Object, kElementsOffset',
     'FixedArray, data, uintptr_t, kHeaderSize',
     'Map, instance_attributes, int, kInstanceAttributesOffset',
-    'Map, inobject_properties, int, kInObjectPropertiesOffset',
+    'Map, inobject_properties_of_constructor_function_index_offset, int, kInObjectPropertiesOrConstructorFunctionIndexOffset',
     'Map, instance_size, int, kInstanceSizeOffset',
     'Map, bit_field, char, kBitFieldOffset',
     'Map, bit_field2, char, kBitField2Offset',