[Windows] Add helper for headless integration tests (#50885)

We'll add more headless mode tests as part of flutter/flutter#143765.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

shell/platform/windows/flutter_windows_unittests.cc

@@ -116,10 +116,8 @@ TEST_F(WindowsTest, LaunchCustomEntrypointInEngineRunInvocation) {
 TEST_F(WindowsTest, LaunchHeadlessEngine) {
   auto& context = GetContext();
   WindowsConfigBuilder builder(context);
-  EnginePtr engine{builder.InitializeEngine()};
+  EnginePtr engine{builder.RunHeadless()};
   ASSERT_NE(engine, nullptr);
-
-  ASSERT_TRUE(FlutterDesktopEngineRun(engine.get(), nullptr));
 }
 
 // Verify that accessibility features are initialized when a view is created.

shell/platform/windows/testing/windows_test_config_builder.cc

@@ -68,7 +68,28 @@ FlutterDesktopEngineProperties WindowsConfigBuilder::GetEngineProperties()
 
 EnginePtr WindowsConfigBuilder::InitializeEngine() const {
   FlutterDesktopEngineProperties engine_properties = GetEngineProperties();
-  return EnginePtr(FlutterDesktopEngineCreate(&engine_properties));
+  return EnginePtr{FlutterDesktopEngineCreate(&engine_properties)};
+}
+
+EnginePtr WindowsConfigBuilder::RunHeadless() const {
+  InitializeCOM();
+
+  EnginePtr engine = InitializeEngine();
+  if (!engine) {
+    return {};
+  }
+
+  // Register native functions.
+  FlutterWindowsEngine* windows_engine =
+      reinterpret_cast<FlutterWindowsEngine*>(engine.get());
+  windows_engine->SetRootIsolateCreateCallback(
+      context_.GetRootIsolateCallback());
+
+  if (!FlutterDesktopEngineRun(engine.get(), /* entry_point */ nullptr)) {
+    return {};
+  }
+
+  return engine;
 }
 
 ViewControllerPtr WindowsConfigBuilder::Run() const {
@@ -87,8 +108,8 @@ ViewControllerPtr WindowsConfigBuilder::Run() const {
 
   int width = 600;
   int height = 400;
-  ViewControllerPtr controller(
-      FlutterDesktopViewControllerCreate(width, height, engine.release()));
+  ViewControllerPtr controller{
+      FlutterDesktopViewControllerCreate(width, height, engine.release())};
   if (!controller) {
     return {};
   }

shell/platform/windows/testing/windows_test_config_builder.h

@@ -64,8 +64,16 @@ class WindowsConfigBuilder {
   // Returns a configured and initialized engine.
   EnginePtr InitializeEngine() const;
 
-  // Returns a configured and initialized view controller running the default
-  // Dart entrypoint.
+  // Returns a configured and initialized engine that runs the configured Dart
+  // entrypoint.
+  //
+  // Returns null on failure.
+  EnginePtr RunHeadless() const;
+
+  // Returns a configured and initialized view controller that runs the
+  // configured Dart entrypoint.
+  //
+  // Returns null on failure.
   ViewControllerPtr Run() const;
 
  private: