Browser+LibWeb+WebContent: Track the source of document.cookie requests

To implement the HttpOnly attribute, the CookieJar needs to know where a
request originated from. Namely, it needs to distinguish between HTTP /
non-HTTP (i.e. JavaScript) requests. When the HttpOnly attribute is set,
requests from JavaScript are to be blocked.

Userland/Libraries/LibWeb/Cookie/Cookie.h

@@ -31,6 +31,11 @@
 
 namespace Web::Cookie {
 
+enum class Source {
+    NonHttp,
+    Http,
+};
+
 struct Cookie {
     String name;
     String value;