We found why we can playback the purchased videos of YTTV, but cannot test EME website. Because SourceBuffer::appendBuffer is used in 2017 EME test website, but YTTV home page can automatically select the old API if SourceBuffer::appendBuffer does not be implemented.
EME 2017 test website: http://yt-dash-mse-test.commondatastorage.googleapis.com/unit-tests/2017.html?test_type=encryptedmedia-test
Cobalt team is aware of the need to upgrade the supported EME version, see https://stackoverflow.com/a/41070502/1318869.
In the meanwhile, for the purpose of testing of Starboard player implementation, try running youtube.com/tv in Cobalt and play any purchased video.