LibWeb: Implement XMLHttpRequest.overrideMimeType

This allows you to ignore the Content-Type returned by the server and
always parse the content as if it's the given MIME type.

This will currently be used for allowing you to override the charset
of text responses.

Userland/Libraries/LibWeb/XHR/XMLHttpRequest.h

@@ -14,6 +14,7 @@
 #include <LibWeb/Bindings/Wrappable.h>
 #include <LibWeb/DOM/EventTarget.h>
 #include <LibWeb/DOM/ExceptionOr.h>
+#include <LibWeb/MimeSniff/MimeType.h>
 #include <LibWeb/XHR/XMLHttpRequestEventTarget.h>
 
 namespace Web::XHR {
@@ -62,6 +63,8 @@ public:
     Bindings::CallbackType* onreadystatechange();
     void set_onreadystatechange(Optional<Bindings::CallbackType>);
 
+    DOM::ExceptionOr<void> override_mime_type(String const& mime);
+
 private:
     virtual void ref_event_target() override { ref(); }
     virtual void unref_event_target() override { unref(); }
@@ -91,6 +94,9 @@ private:
     bool m_timed_out { false };
 
     ByteBuffer m_response_object;
+
+    // https://xhr.spec.whatwg.org/#override-mime-type
+    Optional<MimeSniff::MimeType> m_override_mime_type;
 };
 
 }