Reduce noise on the memory benchmarks (#19630)

- Check memory usage in release builds, not profile.
- Use multiple runs and average the results.

dev/benchmarks/complex_layout/test_memory/scroll_perf.dart

@@ -0,0 +1,72 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+
+import 'package:complex_layout/main.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter/scheduler.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+/// The speed, in pixels per second, that the drag gestures should end with.
+const double speed = 1500.0;
+
+/// The number of down drags and the number of up drags. The total number of
+/// gestures is twice this number.
+const int maxIterations = 4;
+
+/// The time that is allowed between gestures for the fling effect to settle.
+const Duration pauses = Duration(milliseconds: 500);
+
+Future<void> main() async {
+  final Completer<void> ready = new Completer<void>();
+  runApp(new GestureDetector(
+    onTap: () {
+      debugPrint('Received tap.');
+      ready.complete();
+    },
+    behavior: HitTestBehavior.opaque,
+    child: new IgnorePointer(
+      ignoring: true,
+      child: new ComplexLayoutApp(),
+    ),
+  ));
+  await SchedulerBinding.instance.endOfFrame;
+
+  /// Wait 50ms to allow the GPU thread to actually put up the frame. (The
+  /// endOfFrame future ends when we send the data to the engine, before the GPU
+  /// thread has had a chance to rasterize, etc.)
+  await new Future<Null>.delayed(const Duration(milliseconds: 50));
+  debugPrint('==== MEMORY BENCHMARK ==== READY ====');
+
+  await ready.future; // waits for tap sent by devicelab task
+  debugPrint('Continuing...');
+
+  // remove onTap handler, enable pointer events for app
+  runApp(new GestureDetector(
+    child: new IgnorePointer(
+      ignoring: false,
+      child: new ComplexLayoutApp(),
+    ),
+  ));
+  await SchedulerBinding.instance.endOfFrame;
+
+  final WidgetController controller = new LiveWidgetController(WidgetsBinding.instance);
+
+  // Scroll down
+  for (int iteration = 0; iteration < maxIterations; iteration += 1) {
+    debugPrint('Scroll down... $iteration/$maxIterations');
+    await controller.fling(find.byType(ListView), const Offset(0.0, -700.0), speed);
+    await new Future<Null>.delayed(pauses);
+  }
+
+  // Scroll up
+  for (int iteration = 0; iteration < maxIterations; iteration += 1) {
+    debugPrint('Scroll up... $iteration/$maxIterations');
+    await controller.fling(find.byType(ListView), const Offset(0.0, 300.0), speed);
+    await new Future<Null>.delayed(pauses);
+  }
+
+  debugPrint('==== MEMORY BENCHMARK ==== DONE ====');
+}