Reduce noise on complex_layout_scroll_perf__timeline_summary (#20400)

See https://github.com/flutter/flutter/issues/19434

dev/benchmarks/complex_layout/test_driver/scroll_perf_test.dart

@@ -20,19 +20,24 @@ void main() {
     });
 
     Future<Null> testScrollPerf(String listKey, String summaryName) async {
+      // The slight initial delay avoids starting the timing during a
+      // period of increased load on the device. Without this delay, the
+      // benchmark has greater noise.
+      // See: https://github.com/flutter/flutter/issues/19434
+      await new Future<Null>.delayed(const Duration(milliseconds: 250));
       final Timeline timeline = await driver.traceAction(() async {
         // Find the scrollable stock list
         final SerializableFinder list = find.byValueKey(listKey);
         expect(list, isNotNull);
 
         // Scroll down
-        for (int i = 0; i < 5; i++) {
+        for (int i = 0; i < 5; i += 1) {
           await driver.scroll(list, 0.0, -300.0, const Duration(milliseconds: 300));
           await new Future<Null>.delayed(const Duration(milliseconds: 500));
         }
 
         // Scroll up
-        for (int i = 0; i < 5; i++) {
+        for (int i = 0; i < 5; i += 1) {
           await driver.scroll(list, 0.0, 300.0, const Duration(milliseconds: 300));
           await new Future<Null>.delayed(const Duration(milliseconds: 500));
         }