SELECT origin, count(origin) as cnt FROM (
SELECT
JSON_EXTRACT_SCALAR(payload, '$._was_pushed') as pushed,
HOST(JSON_EXTRACT(payload, '$._full_url')) as origin
FROM [httparchive:har.2016_07_01_chrome_requests]
)
WHERE pushed == "1"
GROUP BY origin
ORDER BY cnt DESC
Not a very long list as of 07/16 but here are a few of the top ones: