This adds a command line option to enable the DevTools server. Here, we
make the application the DevToolsDelegate to reply to requests from the
DevTools server about the state of the application.
This commit adds a "echo_server_port" property to `WebContentOptions`.
Additionally, it makes `Application::web_content_options()` return a
mutable reference instead of a const reference so that we can set the
port value from the fixture.
