Update Windows system font change handling (#66021)

The engine now has an explicit API for system font changes, rather than
expecting the WM_FONTCHANGE message to be forwarded to the Flutter view.

packages/flutter_tools/templates/app/windows.tmpl/runner/flutter_window.cpp

@@ -53,5 +53,12 @@ FlutterWindow::MessageHandler(HWND hwnd, UINT const message,
       return *result;
     }
   }
+
+  switch (message) {
+    case WM_FONTCHANGE:
+      flutter_controller_->engine()->ReloadSystemFonts();
+      break;
+  }
+
   return Win32Window::MessageHandler(hwnd, message, wparam, lparam);
 }

packages/flutter_tools/templates/app/windows.tmpl/runner/win32_window.cpp

@@ -187,11 +187,6 @@ Win32Window::MessageHandler(HWND hwnd,
         SetFocus(child_content_);
       }
       return 0;
-
-    // Messages that are directly forwarded to embedding.
-    case WM_FONTCHANGE:
-      SendMessage(child_content_, WM_FONTCHANGE, NULL, NULL);
-      return 0;
   }
 
   return DefWindowProc(window_handle_, message, wparam, lparam);