Update postmortem metadata generation script

This commit updates gen-postmortem-metadata.py to re-export
the v8dbg_class_JSFunction__shared__SharedFunctionInfo
constant.

See: https://github.com/nodejs/node/pull/25852
Change-Id: I60f39c96f3f22d6f10ec38b0af3c975908c7b7f2
Reviewed-on: https://chromium-review.googlesource.com/c/1450144Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59334}

tools/gen-postmortem-metadata.py

@@ -245,6 +245,7 @@ consts_misc = [
 #
 extras_accessors = [
     'JSFunction, context, Context, kContextOffset',
+    'JSFunction, shared, SharedFunctionInfo, kSharedFunctionInfoOffset',
     'HeapObject, map, Map, kMapOffset',
     'JSObject, elements, Object, kElementsOffset',
     'JSObject, internal_fields, uintptr_t, kHeaderSize',