javascript fix on listing downloaded files via selenium

[tstrych]

Automation failure correction of test_positive_export
https://github.com/SatelliteQE/robottelo/blob/master/tests/foreman/ui/test_host.py#L814

export automatically goes to browser where files[0] were empty and this cause an error later on.
https://github.com/SatelliteQE/airgun/blob/master/airgun/browser.py#L756

I looked into the JavaScript and it seems there can be also another option fileUrl.

============================= test session starts ==============================
collected 28 items / 27 deselected / 1 selected
============ 1 passed, 27 deselected, 2 warnings in 165.83 seconds =============

[mirekdlugosz]

We had #323 opened a long time ago, but I like this approach a little better, as it keeps backwards compatibility with very old Chrome versions.

ACK

[omkarkhatavkar]

ACK, +1 gives on to a backward compatibility

[tstrych]

Can you @mirekdlugosz merge this ? I have two ACKs. I shoudn't merge it as I am not tier* reviewer for airgun.