iOS: Migrate FlutterHeadlessDartRunner to ARC

This is a subclass of FlutterEngine, which is still manually reference
counted, but that's not problematic since the headers have been migrated
to ARC-safe syntax that preserves existing behaviour.

Issue: flutter/flutter#137801

shell/platform/darwin/ios/BUILD.gn

@@ -70,6 +70,7 @@ source_set("flutter_framework_source_arc") {
     "framework/Source/FlutterDartVMServicePublisher.mm",
     "framework/Source/FlutterEmbedderKeyResponder.h",
     "framework/Source/FlutterEmbedderKeyResponder.mm",
+    "framework/Source/FlutterHeadlessDartRunner.mm",
     "framework/Source/FlutterKeyPrimaryResponder.h",
     "framework/Source/FlutterKeySecondaryResponder.h",
     "framework/Source/FlutterKeyboardManager.h",
@@ -183,7 +184,6 @@ source_set("flutter_framework_source") {
     "framework/Source/FlutterEngine.mm",
     "framework/Source/FlutterEngineGroup.mm",
     "framework/Source/FlutterEngine_Internal.h",
-    "framework/Source/FlutterHeadlessDartRunner.mm",
     "framework/Source/FlutterPlatformPlugin.h",
     "framework/Source/FlutterPlatformPlugin.mm",
     "framework/Source/FlutterViewController.mm",

shell/platform/darwin/ios/framework/Source/FlutterHeadlessDartRunner.mm

@@ -23,8 +23,9 @@
 #import "flutter/shell/platform/darwin/ios/framework/Source/platform_message_response_darwin.h"
 #import "flutter/shell/platform/darwin/ios/platform_view_ios.h"
 
-@implementation FlutterHeadlessDartRunner {
-}
+FLUTTER_ASSERT_ARC
+
+@implementation FlutterHeadlessDartRunner
 
 - (instancetype)initWithName:(NSString*)labelPrefix project:(FlutterDartProject*)projectOrNil {
   return [self initWithName:labelPrefix project:projectOrNil allowHeadlessExecution:YES];
@@ -56,4 +57,5 @@ - (instancetype)initWithName:(NSString*)labelPrefix
 - (instancetype)init {
   return [self initWithName:@"io.flutter.headless" project:nil];
 }
+
 @end