src: update postmortem constant

Update a postmortem constant used by the ustack helper which
changed while moving to V8 7.1.

PR-URL: #23423
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Myles Borins <myles.borins@gmail.com>

src/v8abbr.h

@@ -100,7 +100,7 @@
 #define V8_OFF_SCRIPT_LENDS \
     V8_OFF_HEAP(V8DBG_CLASS_SCRIPT__LINE_ENDS__OBJECT)
 #define V8_OFF_STR_LENGTH \
-    V8_OFF_HEAP(V8DBG_CLASS_STRING__LENGTH__SMI)
+    V8_OFF_HEAP(V8DBG_CLASS_STRING__LENGTH__INT32_T)
 #define V8_OFF_STR_CHARS  \
     V8_OFF_HEAP(V8DBG_CLASS_SEQONEBYTESTRING__CHARS__CHAR)
 #define V8_OFF_CONSSTR_CAR  \