Inspector: omnibus commit for the probe agent.

 * Clean up inspector protocol design for probes.
 * Add an observer and manager for the Probe domain of inspector protocol.
 * Add frontend models for probes and probe samples. (untested)
 * Register the probe agent in the frontend and backend.
 * Fix some obvious crash bugs in the probe server and resolver.

The probe server/resolver do not record probe samples yet, but the probe agent
can be enabled and a new probe can be registered.