[vm] Fix CoreSnapshotSize and StandaloneSnapshotSize to account for t…

…he image page size.

Should have been part of c8f3f13.

Change-Id: I2a346f66490e6e97d846d4646ca7042be0b19cb2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/116101
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/vm/benchmark_test.cc

@@ -547,7 +547,8 @@ static void BenchmarkSnapshotSize(Benchmark* benchmark, const char* script) {
   const Snapshot* snapshot =
       Snapshot::SetupFromBuffer(isolate_snapshot_data_buffer);
   ASSERT(snapshot->kind() == Snapshot::kFull);
-  benchmark->set_score(snapshot->length());
+  benchmark->set_score(writer.IsolateSnapshotSize() +
+                       isolate_image_writer.data_size());
 
   free(vm_snapshot_data_buffer);
   free(vm_snapshot_text_buffer);