LibWeb: Pass cookie string by reference in Document::set_cookie

This string is only taken by const reference internally, so there's no
point in forcing the callers to copy the string.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -1006,7 +1006,7 @@ String Document::cookie(Cookie::Source source)
     return {};
 }
 
-void Document::set_cookie(String cookie_string, Cookie::Source source)
+void Document::set_cookie(String const& cookie_string, Cookie::Source source)
 {
     auto cookie = Cookie::parse_cookie(cookie_string);
     if (!cookie.has_value())

Userland/Libraries/LibWeb/DOM/Document.h

@@ -56,7 +56,7 @@ public:
     virtual ~Document() override;
 
     String cookie(Cookie::Source = Cookie::Source::NonHttp);
-    void set_cookie(String, Cookie::Source = Cookie::Source::NonHttp);
+    void set_cookie(String const&, Cookie::Source = Cookie::Source::NonHttp);
 
     String referrer() const;