LibWeb: Use current platform for navigator.platform

Before, navigator.platform would always report the platform as "Serenity
OS", regardless of whether or not that was true. It also did not include
the architecture, which Firefox and Chrome both do. Now, it can report
either "Linux x86_64" or "SerenityOS AArch64".

Userland/Libraries/LibWeb/Loader/ResourceLoader.cpp

@@ -58,6 +58,7 @@ ErrorOr<NonnullRefPtr<ResourceLoader>> ResourceLoader::try_create(NonnullRefPtr<
 ResourceLoader::ResourceLoader(NonnullRefPtr<ResourceLoaderConnector> connector)
     : m_connector(move(connector))
     , m_user_agent(default_user_agent)
+    , m_platform(default_platform)
 {
 }