Instead of sending a message to the script thread via IPC when a web
font loads and then sending another, just give the `FontContext` a
callback that send a single message to the script thread. This moves all
the cache invalidation internally into `FontContext` as well.
Additionally, the unused LayoutControlMessage::ExitNow enum variant is
removed.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Mukilan Thiyagarajan <mukilan@igalia.com>