[js-perf] Improve end of serialize StackTrace benchmark

The serialize set of microbenchmarks serializes Error.stack of a
prepared set of error objects. When all objects have been serialized,
the benchmark then wrapped around and only accessed the resulting
string properties. This, of course, is a lot faster and benchmark
results are heavily fluctuating when the end is reached.

This CL fixes this by introducing a payload that should be similiar
to the specific workload, which is executed after the end is reached.

R=petermarshall@chromium.org

Bug: v8:8742
Change-Id: I7183d04e7c06af0c16fe3412e902f0d33605bc25
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1524485Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60297}

test/js-perf-test/StackTrace/serialize.js

@@ -71,9 +71,11 @@ function SerializeStack() {
     // Trigger serialization by accessing Error.stack.
     %FlattenString(errorsCreatedBySetup[counter++].stack);
   } else {
-    // The counter is reset after hitting the end, although
-    // Error.stack is cached at this point.
-    counter = 0;
+    // All errors are serialized. The stack trace string is now cached, so
+    // re-iterating the array is a simple property lookup. Instead,
+    // a simple Error object is created and serialized, otherwise the benchmark
+    // result would fluctuate heavily if it reaches the end.
+    %FlattenString(new Error().stack);
   }
 }