LibWeb+WebDriver: Add an IPC-transferable Web::WebDriver::Response class

This is essentially an ErrorOr<JsonValue, Web::WebDriver::Error> class.
Unfortunately, that ErrorOr would not be default-constructible, which is
required for the generated IPC classes. So this is a thin wrapper around
a Variant<JsonValue, Web::WebDriver::Error> to emulate ErrorOr.

Userland/Libraries/LibWeb/WebDriver/Response.cpp

@@ -0,0 +1,76 @@
+/*
+ * Copyright (c) 2022, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibIPC/Decoder.h>
+#include <LibIPC/Encoder.h>
+#include <LibWeb/WebDriver/Response.h>
+
+enum class ResponseType : u8 {
+    Success,
+    Error,
+};
+
+namespace Web::WebDriver {
+
+Response::Response(JsonValue&& value)
+    : m_value_or_error(move(value))
+{
+}
+
+Response::Response(Error&& error)
+    : m_value_or_error(move(error))
+{
+}
+
+}
+
+bool IPC::encode(Encoder& encoder, Web::WebDriver::Response const& response)
+{
+    response.visit(
+        [](Empty) { VERIFY_NOT_REACHED(); },
+        [&](JsonValue const& value) {
+            encoder << ResponseType::Success;
+            encoder << value;
+        },
+        [&](Web::WebDriver::Error const& error) {
+            encoder << ResponseType::Error;
+            encoder << error.http_status;
+            encoder << error.error;
+            encoder << error.message;
+            encoder << error.data;
+        });
+
+    return true;
+}
+
+ErrorOr<void> IPC::decode(Decoder& decoder, Web::WebDriver::Response& response)
+{
+    ResponseType type {};
+    TRY(decoder.decode(type));
+
+    switch (type) {
+    case ResponseType::Success: {
+        JsonValue value;
+        TRY(decoder.decode(value));
+
+        response = move(value);
+        break;
+    }
+
+    case ResponseType::Error: {
+        Web::WebDriver::Error error {};
+        TRY(decoder.decode(error.http_status));
+        TRY(decoder.decode(error.error));
+        TRY(decoder.decode(error.message));
+        TRY(decoder.decode(error.data));
+
+        response = move(error);
+        break;
+    }
+    }
+
+    return {};
+}