Instead of passing RequestServer and ImageDecoder socket FDs as
command-line arguments to WebContent, send them over the main IPC
channel after launch. This unifies initial connection and reconnection
into a single code path.
Add IPC::TransportHandle as an abstraction for passing IPC
transports through .ipc messages. This replaces IPC::File at
all sites where a transport (not a generic file) is being
transferred between processes.
TransportHandle provides from_transport(),
clone_from_transport(), and create_transport() methods that
encapsulate the fd-to-socket-to-transport conversion in one
place. This is preparatory work for Mach port support on
macOS -- when that lands, only TransportHandle's internals
need to change while all .ipc definitions and call sites
remain untouched.
Since cross-site navigation is a pretty frequent task, creating a spare
process is commonplace in other browsers to reduce the overhead of
directing the target site to a new process.
We store this process on the WebView application. If it is unavailable,
we queue a task to create it later.
LibWebView now knows how to launch RequestServer and ImageDecoderServer
without help from the UI, so let's move ownership of these services over
to LibWebView for de-duplication.
