Temporarily restrict kVTVideoEncoderSpecification_RequiredLowLatency use to iOS
<rdar://problem/61191539>
Created attachment 395263 [details] Patch
Committed r259385: <https://trac.webkit.org/changeset/259385> All reviewed patches have been landed. Closing bug and clearing flags on attachment 395263 [details].
Followup build fix in r259397