LibJS+LibWeb: Pass prototype to Object constructor

Everyone who constructs an Object must now pass a prototype object when
applicable. There's still a fair amount of code that passes something
fetched from the Interpreter, but this brings us closer to being able
to detach prototypes from Interpreter eventually.

Libraries/LibJS/Runtime/StringObject.cpp

@@ -41,9 +41,9 @@ StringObject* StringObject::create(GlobalObject& global_object, PrimitiveString&
 }
 
 StringObject::StringObject(PrimitiveString& string, Object& prototype)
-    : m_string(string)
+    : Object(&prototype)
+    , m_string(string)
 {
-    set_prototype(&prototype);
 }
 
 StringObject::~StringObject()