Sky debugger is a native mojo app that puts up an http server and talks
to other mojo apps in response to http requests, allowing to
interactively start/stop tracing, profile, etc.
This is generally useful also when one does not run mojo_shell through
skydb, hence this patch pulls the debugger into services/debugger.
We can then add plumbing for running with the debugger in devtools.
Bug: domokit/devtools/#4.
R=abarth@chromium.org
Review URL: https://codereview.chromium.org/1151573010