change the `experimental.getDiagnostics` call from the experimental domain to a `diagnostics` domain

[devoncarew]

The diagnostics domain can still be marked as experimental, but we'll need it to settle in it's final location in order to start calling it from the client. So, diagnostics.getDiagnostics. The diagnostics domain may or may not get logging capability.

[pq]

https://codereview.chromium.org/1453433002/

[devoncarew]

Thanks!

[pq]

Done w/ 6d9cb5e.