[DevTools] Plumb mojom::DevToolsAgentHost around

This is a preparation to worker debugging over mojom::DevToolsAgent
interface. To properly report child workers and establish connection
to them, we need a host which will be notified about children.

For now, just plumb an empty interface to separate concerns.
See https://chromium-review.googlesource.com/c/chromium/src/+/1183589
for the full implementation.

Bug: 775132
Change-Id: Iac8b94309cc431baf18871b3355d817b326e51ac
Reviewed-on: https://chromium-review.googlesource.com/c/1265030
Reviewed-by: Andrey Kosyakov <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Commit-Queue: Dmitry Gozman <[email protected]>
Cr-Commit-Position: refs/heads/master@{#598535}
41 files changed