deps: cherry-pick 1aead19 from upstream V8

Original commit message:

    Add postmortem metadata for thin strings.

    See: nodejs/llnode#117
    Change-Id: Icc2830c8e9096610df33ffdc2f89e74cb1b35662
    Reviewed-on: https://chromium-review.googlesource.com/618986
    Reviewed-by: Michael Achenbach <machenbach@chromium.org>
    Commit-Queue: Ben Noordhuis <info@bnoordhuis.nl>
    Cr-Commit-Position: refs/heads/master@{nodejs#47778}

PR-URL: nodejs#15184
Ref: nodejs/llnode#117
Ref: nodejs/llnode#121
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

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

@@ -72,6 +72,7 @@
     { 'name': 'ConsStringTag',          'value': 'kConsStringTag' },
     { 'name': 'ExternalStringTag',      'value': 'kExternalStringTag' },
     { 'name': 'SlicedStringTag',        'value': 'kSlicedStringTag' },
+    { 'name': 'ThinStringTag',          'value': 'kThinStringTag' },
 
     { 'name': 'HeapObjectTag',          'value': 'kHeapObjectTag' },
     { 'name': 'HeapObjectTagMask',      'value': 'kHeapObjectTagMask' },