Tentatively assigning to treib@, and I see he's touched multiple callsites with this function.  Feel free to reassign as appropriate.

It's possible this may have already been handled by the fix for  bug 120657  (which was in bug 665624).  It's not clear to me at a glance.  (Maria implies it was not, but she may have forgotten the details in between her submitting a change and me asking her about it six months later.)

I checked the code: No, it's not already fixed, but it should be straightforward to apply the TLD check also to IsYoutubeDomainUrl, assuming that the list of valid TLDs is the same for both youtube and google hosts.

I don't think it's safe to assume that TLDs are the same. However, we do have a script (https://docs.google.com/document/d/1haYVf-NCkbiaDcbOf7Djx_jlqLXcSr-Op86eHXV-yi4/edit#bookmark=id.hu8i66if36k9) that parses domain file where all Google domains are registered to extract TLDs for search and it would be trivial to modify it to do the same for YouTube.