Expose the remote debugger URL for content shell

[nex3]

In addition to Observatory (#295), it may be useful to inspect the HTML contents of the page with content shell. To enable this, we should enable its remote debugger and print out the debugger's URL when the test runner is paused for debugging (as in #294).

This is complicated by the fact that content shell doesn't support accessing an auto-allocated port. To work around this, we'll have to do some sort of manual port allocation and collision recovery. Luckily, content shell does print a recognizable error message when its call to bind() fails:

[31788:31805:0722/150225:79307133024:ERROR:socket_libevent.cc(122)] bind() returned an error, errno=98: Address already in use
[31788:31805:0722/150225:79307133110:ERROR:devtools_http_handler_impl.cc(728)] Cannot start http server for devtools. Stop devtools.