LibWeb: Implement 'Default User-Agent value' AO

Author: https://github.com/linusg
Commit: 6deb5ce9b5
Pull-request: https://github.com/SerenityOS/serenity/pull/15795
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/davidot ✅

Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.cpp

@@ -17,6 +17,7 @@
 #include <LibWeb/Fetch/Infrastructure/HTTP.h>
 #include <LibWeb/Fetch/Infrastructure/HTTP/Headers.h>
 #include <LibWeb/Infra/ByteSequences.h>
+#include <LibWeb/Loader/ResourceLoader.h>
 #include <LibWeb/MimeSniff/MimeType.h>
 
 namespace Web::Fetch::Infrastructure {
@@ -686,4 +687,11 @@ Optional<RangeHeaderValue> parse_single_range_header_value(ReadonlyBytes value)
     return RangeHeaderValue { move(range_start_value), move(range_end_value) };
 }
 
+// https://fetch.spec.whatwg.org/#default-user-agent-value
+ErrorOr<ByteBuffer> default_user_agent_value()
+{
+    // A default `User-Agent` value is an implementation-defined header value for the `User-Agent` header.
+    return ByteBuffer::copy(default_user_agent.bytes());
+}
+
 }