headless-browser currently uses its own PageClient to load web pages
in-process. Due to this, it also needs to set up a whole bunch of other
objects needed to run LibWeb, e.g. image decoders, request servers, etc.
This changes headless-browser to instead implement a WebView to launch
WebContent out-of-process. This implementation is almost entirely empty,
but can be filled in as-needed. For example, we may want to print
JavaScript console messages.
