https://chromium-review.googlesource.com/c/578482 plumbs URLs from RenderFrameHostImpl to a coordination unit. While this works, this is not ideal for several reasons:
- It requires duplicating the URL from //content into //services/resource_coordinator. Since GRC currently lives in the same process, this requires at a number of string copies.
- It eliminates a single source of truth for the URL of a frame. Navigation is quite complex (for example, we don't get document.open() quite right today), and now we have to make sure an additional location is updated.
- Coordination units can be accessed by any process (by design). An untrusted process can spoof this property.
- While GRC currently lives in the browser process, there's no guarantee it will remain so in the future. We definitely want to make sure URLs don't leak into an untrusted process if this changes.

erikchen@ makes a good point that the old tracing code embedded URLs as well though *shrug*