When looking through open export PRs (https://github.com/w3c/web-platform-tests/labels/chromium-export), I noticed a couple old PRs that no longer correspond to exportable CLs, because:
 - the CL was abandoned, OR
 - all wpt changes were removed from the CL.

This is because currently, GerritAPI.query_exportable_open_cls returns only open CLs (not including abandoned ones) with changes in wpt (exportable), so by the time we get to TestExporter.process_gerrit_cls, those CLs aren't in the list and corresponding PRs aren't updated.

One possible change here would be to also loop through open PRs (https://github.com/w3c/web-platform-tests/labels/chromium-export), and for each one, get the corresponding CL and check whether it's still an open and exportable CL. If not, then we can send a request to GitHub to close the PR.