Embed Flutter DevTools in Chrome DevTools

[elliette]

Copied from flutter/devtools#2930, since this is in fact a Dart Debug Extension request.

Flutter DevTools embedded in Chrome DevTools is supported internally, but not externally. This would required updating Flutter's response headers to include the origin-agent-cluster.

See: https://web.dev/origin-agent-cluster/#why-browsers-can't-automatically-segregate-same-site-origins

[bkonyi]

Action items:

• Update flutter_tools and webdev to update the response headers to include origin-agent-cluster
• Remove internal / external check in the debug extension to enable for all users