Add 32-bit iOS gallery startup test (#17653)

dev/devicelab/bin/tasks/flutter_gallery_ios32__start_up.dart

+// 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:flutter_devicelab/tasks/perf_tests.dart';
+import 'package:flutter_devicelab/framework/adb.dart';
+import 'package:flutter_devicelab/framework/framework.dart';
+
+Future<Null> main() async {
+  deviceOperatingSystem = DeviceOperatingSystem.ios;
+  await task(createFlutterGalleryStartupTest());
+}

dev/devicelab/manifest.yaml

@@ -440,6 +440,13 @@ tasks:
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
 
+  flutter_gallery_ios32__start_up:
+    description: >
+      Measures the startup time of the Flutter Gallery app on 32-bit iOS.
+    stage: devicelab_ios
+    required_agent_capabilities: ["mac/ios32"]
+    flaky: true
+
   flutter_gallery_ios32__transition_perf:
     description: >
       Measures the performance of screen transitions in Flutter Gallery on