Reduce memory benchmark perf tests outer loop to 10. (#28281)

* reduce iterations to avoid test timing out

dev/devicelab/lib/tasks/perf_tests.dart

@@ -412,7 +412,7 @@ class MemoryTest {
     _receivedNextMessage = Completer<void>();
   }
 
-  int get iterationCount => 15;
+  int get iterationCount => 10;
 
   Device get device => _device;
   Device _device;