Add iOS backdrop filter benchmarks (#38814)

For https://github.com/flutter/flutter/issues/36064

That performance regression is iOS-only so we'd better get the
benchmarks running on iOS.

dev/devicelab/bin/tasks/backdrop_filter_perf_ios__timeline_summary.dart

+// Copyright 2019 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<void> main() async {
+  deviceOperatingSystem = DeviceOperatingSystem.ios;
+  await task(createBackdropFilterPerfTest());
+}

dev/devicelab/manifest.yaml

@@ -431,6 +431,13 @@ tasks:
     stage: devicelab_ios
     required_agent_capabilities: ["mac/ios"]
 
+  backdrop_filter_perf_ios__timeline_summary:
+    description: >
+      Measures the runtime performance of backdrop filter blurs on iOS.
+    stage: devicelab_ios
+    required_agent_capabilities: ["mac/ios"]
+    flaky: true
+
   complex_layout_scroll_perf_ios__timeline_summary:
     description: >
       Measures the runtime performance of the Complex Layout sample app on