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.
