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chromium/chrome/test/data/page_load_metrics/use_counter_features_in_iframes.html 

<html><head><title>iframe test</title></head>
<body>
<iframe id="f0" allow="fullscreen 'none'" src="use_counter_features.html"></iframe>
<iframe id="f1" allow="fullscreen 'none'" src="http://localhost:8000/use_counter_features.html"></iframe>
</body></html>