SiteInstance::GetSiteForURL must run in UI thread, but we call it in IO thread in TorProfileService::SetProxy

[riastradh-brave]

As of chromium/chromium@ea6921f (2019-01-29), SiteInstance::GetSiteForURL fails DCHECKs if we run it in the IO thread, but we do exactly that when we call TorProfileService::SetProxy in an OnBeforeURLRequest handler:

int TorProfileServiceImpl::SetProxy(net::ProxyResolutionService* service,
                                    const GURL& request_url,
                                    bool new_circuit) {
  DCHECK_CURRENTLY_ON(BrowserThread::IO);
  const TorConfig tor_config = tor_launcher_factory_->GetTorConfig();
  GURL url = SiteInstance::GetSiteForURL(profile_, request_url);

https://github.com/brave/brave-core/blob/b4afcc7dd690ee4393823c3d9fa6d9ea94c455a2/browser/tor/tor_profile_service_impl.cc#L97-L102

This breaks debug builds. It may also have other consequences in non-debug builds -- I haven't seen symptoms but I haven't ruled them out either.

It is not immediately clear to me how to resolve this. Internally, SiteInstanceImpl::GetSiteForURL is allowed in the IO thread when we have a BrowserOrResourceContext and should_use_effective_urls is false. Should we use SiteInstanceImpl::DetermineProcessLockURL, with some isolation context? Should we use the internal SiteInstanceImpl::GetSiteForURL directly?

cc @darkdh, @bridiver