[postmortem] add JS_ERROR_TYPE and context embedder index

  * JS_ERROR_TYPE is required for postmortem tools to inspect JSError
    objects (see https://github.com/nodejs/llnode/pull/215 for a usage
    example)
  * The context embedder index is required for postmortem tools to
    access embedder data stored in the context (see
    https://github.com/nodejs/llnode/pull/204 for a usage example)

R=bmeurer@google.com, yangguo@google.com

Change-Id: Ib7c7eb44f6ad327fc71a1d45f510c49377db7a25
Reviewed-on: https://chromium-review.googlesource.com/1138493Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54475}

tools/gen-postmortem-metadata.py

@@ -204,6 +204,9 @@ consts_misc = [
         'value': 'Context::EXTENSION_INDEX' },
     { 'name': 'context_min_slots',
         'value': 'Context::MIN_CONTEXT_SLOTS' },
+    { 'name': 'context_idx_embedder_data',
+        'value': 'Internals::kContextEmbedderDataIndex' },
+
 
     { 'name': 'namedictionaryshape_prefix_size',
         'value': 'NameDictionaryShape::kPrefixSize' },
@@ -224,6 +227,8 @@ consts_misc = [
         'value': 'SimpleNumberDictionaryShape::kPrefixSize' },
     { 'name': 'simplenumberdictionaryshape_entry_size',
         'value': 'SimpleNumberDictionaryShape::kEntrySize' },
+
+    { 'name': 'type_JSError__JS_ERROR_TYPE', 'value': 'JS_ERROR_TYPE' },
 ];
 
 #