src: use HeapStatistics to get external memory

V8 is improving the way external memory is internally managed and how
it is reported.
External memory should now be retrieved using HeapStatistics.

Refs: v8#80

PR-URL: #22754
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/node_process.cc

@@ -208,7 +208,7 @@ void MemoryUsage(const FunctionCallbackInfo<Value>& args) {
   fields[0] = rss;
   fields[1] = v8_heap_stats.total_heap_size();
   fields[2] = v8_heap_stats.used_heap_size();
-  fields[3] = isolate->AdjustAmountOfExternalAllocatedMemory(0);
+  fields[3] = v8_heap_stats.external_memory();
 }
 
 // Most of the time, it's best to use `console.error` to write