Specify browsing context discovery

[jgraham]

We need a mechanism to discover which top level / other browsing contexts exist and a set of events for the browsing context lifecycle (created/destroyed/etc.).

[shs96c]

The mechanism to discover top-level browsing contexts maps cleanly to Get Window Handles.

[k7z45]

From ChromeDriver side, suppose DevTools is listening on a specific port say 64266.
We would first query http://localhost:64266/json/list via http for the top level browsing contexts and then we have a
navigation tracker that would process the events on the same port via WebSocket for the frame related events.
For example, for child browsing context, we can listen for Page.frameAttached . The query plus the events should provide information for both top level and child browsing contexts. However, ChromeDriver does reply on continuously querying the above "json/list" for updating list of active top level browsing contexts.

Side note: I am not sure if there is an equivalent CDP command for querying http://localhost:64266/json/list via WebSocket(maybe Target.getTargets?) or if there is events we could listen for for top level browsing contexts.

[foolip]

This is being done in #62.

[foolip]

Let's called this fixed by #62.