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