GitLab

This CL:
- adds service-side DevTools agent representation (DevToolsAgentHost) and registration for DevTools agents.
- supports requesting the list of available agents using HTTP request and communicating with them using WebSocket.

BUG=478249
TEST=None

Review URL: https://codereview.chromium.org/1164383002

Cr-Commit-Position: refs/heads/master@{#333804}