Allows origin-keyed agent clustering by default.

This policy allows origin-keyed agent clustering by default.

The Origin-Agent-Cluster HTTP header controls whether a document is
isolated in an origin-keyed agent cluster, or in a site-keyed agent
cluster. This has security implications since an origin-keyed agent
cluster allows isolating documents by origin. The developer-visible
consequence of this is that the document.domain accessor can no longer
be set.

The default behaviour - when no Origin-Agent-Cluster header has been set -
changes in M111 from site-keyed to origin-keyed.

If this policy is enabled or not set, the browser will follow this
new default from that version on.

If this policy is disabled this change is reversed and
documents without Origin-Agent-Cluster headers will be assigned to
site-keyed agent clusters. As a consequence, the document.domain accessor
remains settable by default. This matches the legacy behaviour.

See https://developer.chrome.com/blog/immutable-document-domain/ for
additional details.