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.
Temporarily use my commit of libchromiumcontent.