MediaPlayerPrivateAVFoundationObjC::checkPlayability uses dispatch_async(dispatch_get_main_queue()) to trampoline an AVFoundation callback to the "main" thread, but this doesn't work in WK1 where we need to make all calls on the WebThread.
Created attachment 227329 [details] Proposed patch.
Comment on attachment 227329 [details] Proposed patch. r=me
Committed r165998: https://trac.webkit.org/r165998
<rdar://problem/16396919>