deps: cherry-pick 804a693 from upstream V8

Original commit message:

    [postmortem] add JS_ERROR_TYPE and context embedder index

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

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

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

Refs: v8/v8@804a693

PR-URL: nodejs#21855
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yang Guo <yangguo@chromium.org>

common.gypi

@@ -28,7 +28,7 @@
 
     # Reset this number to 0 on major V8 upgrades.
     # Increment by one for each non-official patch applied to deps/v8.
-    'v8_embedder_string': '-node.11',
+    'v8_embedder_string': '-node.12',
 
     # Enable disassembler for `--print-code` v8 options
     'v8_enable_disassembler': 1,

deps/v8/tools/gen-postmortem-metadata.py

@@ -211,6 +211,9 @@
         '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' },
@@ -231,6 +234,8 @@
         'value': 'SimpleNumberDictionaryShape::kPrefixSize' },
     { 'name': 'simplenumberdictionaryshape_entry_size',
         'value': 'SimpleNumberDictionaryShape::kEntrySize' },
+
+    { 'name': 'type_JSError__JS_ERROR_TYPE', 'value': 'JS_ERROR_TYPE' },
 ];
 
 #