LibPDF: Convert PDF::Parser::m_document from RefPtr to WeakPtr

Otherwise both `PDF::Document` and `PDF::Parser` have a `RefPtr`
pointing to each other which leads to a memory leak due to a circular
dependency.

Userland/Libraries/LibPDF/Parser.cpp

@@ -39,7 +39,7 @@ Parser::Parser(ReadonlyBytes bytes)
 {
 }
 
-void Parser::set_document(RefPtr<Document> const& document)
+void Parser::set_document(WeakPtr<Document> const& document)
 {
     m_document = document;
 }