deps: update V8 postmortem metadata script

This commit updates V8's gen-postmortem-metadata.py script
to fix SmartOS compilation for V8 8.4.

Backport-PR-URL: #34356
PR-URL: #33579
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>

common.gypi

@@ -36,7 +36,7 @@
 
     # 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',
 
     ##### V8 defaults for Node.js #####
 

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

@@ -279,6 +279,7 @@
     'SlicedString, offset, SMI, kOffsetOffset',
     'ThinString, actual, String, kActualOffset',
     'Symbol, name, Object, kDescriptionOffset',
+    'FixedArrayBase, length, SMI, kLengthOffset',
 ];
 
 #