Use the "chrome-devtools://" scheme to open devtools.
Allow file scheme in embedded front-end by default.
Setup devtools in the correct phase.

* RegisterDevToolsClientHostFor should be called right after the agent_host and
  the frontend host are created.
* SetupDevToolsFrontendClient should be called before the devtools window is
  going to start navigation.
Cleanup unused headers, data members and functions.
Adopt to API changes of latest URLDataSource.