Function extend_headers 

pub fn extend_headers(
    header_map: &mut HeaderMap,
    user_agent: &str,
    headers: &Option<Box<SerializableHeaderMap>>,
    hostname: &Option<&str>,
    viewport: &Option<Viewport>,
    domain_parsed: &Option<Box<Url>>,
)

Available on crate feature real_browser only.

Extend the headers.