chore: [gn] fix link errors relating to static members (#13488)

I'm not 100% sure why this is working in the gyp build, but I was getting link errors with these variables being defined in the headers.

brightray/browser/win/win32_desktop_notifications/toast.h

@@ -64,7 +64,7 @@ class DesktopNotificationController::Toast {
   float AnimateStackCollapse();
 
  private:
-  static constexpr const TCHAR class_name_[] = TEXT("DesktopNotificationToast");
+  static const TCHAR class_name_[];
 
   const HWND hwnd_;
   HDC hdc_;