Quite a few things can be simplified once request_initiator_*site*_lock should eventually become request_initiator_*origin*_lock.  For example:
- //services/network/initiator_lock_compatibility.cc won't need to know anything about sites (which are more of a //content-level concept anyway).
- Some tests under external/wpt can be made cross-origin instead of cross-site

This is currently blocked on:

- Falling back to site-URL / process-lock in RenderProcessHostImpl::CreateURLLoaderFactoryForRendererProcess.  The work to remove this is mostly blocked by issue 891872.

- Falling back to site-URL in GetOriginForURLLoaderFactory in //content/browser/frame_host/render_frame_host_impl.cc.  This fallback may be removed once the browser always knows the origin to commit (issue 888079).