Fix parameter type in FlutterDesktopEngineCreate (flutter#25439)

FlutterDesktopEngineCreate is part of our C API. We were using a C++
reference type instead of a C-compatible pointer type.

This is a breaking change to anyone calling this directly; we believe
this should affect few people because the Windows template only uses the
`FlutterEngine` wrapper in
`shell/platform/windows/client_wrapper/include/flutter/flutter_engine.h`.

Fixes flutter#75465

Author: cbracken

shell/platform/windows/client_wrapper/flutter_engine.cc

@@ -30,7 +30,7 @@ FlutterEngine::FlutterEngine(const DartProject& project) {
   c_engine_properties.dart_entrypoint_argv =
       entrypoint_argv.size() > 0 ? entrypoint_argv.data() : nullptr;
 
-  engine_ = FlutterDesktopEngineCreate(c_engine_properties);
+  engine_ = FlutterDesktopEngineCreate(&c_engine_properties);
 
   auto core_messenger = FlutterDesktopEngineGetMessenger(engine_);
   messenger_ = std::make_unique<BinaryMessengerImpl>(core_messenger);

shell/platform/windows/client_wrapper/testing/stub_flutter_windows_api.cc

@@ -82,9 +82,9 @@ bool FlutterDesktopViewControllerHandleTopLevelWindowProc(
 }
 
 FlutterDesktopEngineRef FlutterDesktopEngineCreate(
-    const FlutterDesktopEngineProperties& engine_properties) {
+    const FlutterDesktopEngineProperties* engine_properties) {
   if (s_stub_implementation) {
-    return s_stub_implementation->EngineCreate(engine_properties);
+    return s_stub_implementation->EngineCreate(*engine_properties);
   }
   return nullptr;
 }

shell/platform/windows/flutter_windows.cc

@@ -82,8 +82,8 @@ void FlutterDesktopViewControllerForceRedraw(
 }
 
 FlutterDesktopEngineRef FlutterDesktopEngineCreate(
-    const FlutterDesktopEngineProperties& engine_properties) {
-  flutter::FlutterProjectBundle project(engine_properties);
+    const FlutterDesktopEngineProperties* engine_properties) {
+  flutter::FlutterProjectBundle project(*engine_properties);
   auto engine = std::make_unique<flutter::FlutterWindowsEngine>(project);
   return HandleForEngine(engine.release());
 }

shell/platform/windows/public/flutter_windows.h

@@ -131,9 +131,10 @@ FLUTTER_EXPORT bool FlutterDesktopViewControllerHandleTopLevelWindowProc(
 // Creates a Flutter engine with the given properties.
 //
 // The caller owns the returned reference, and is responsible for calling
-// FlutterDesktopEngineDestroy.
+// FlutterDesktopEngineDestroy. The lifetime of |engine_properties| is required
+// to extend only until the end of this call.
 FLUTTER_EXPORT FlutterDesktopEngineRef FlutterDesktopEngineCreate(
-    const FlutterDesktopEngineProperties& engine_properties);
+    const FlutterDesktopEngineProperties* engine_properties);
 
 // Shuts down and destroys the given engine instance. Returns true if the
 // shutdown was successful, or if the engine was not running.