Find nsIDocument via mGlobal (as nsPIDOMWindow) rather than via pointer to
HTMLTrackElement. (bz from #content has explained how we can access
nsPIDOMWindow and nsIDocument without having to ask the HTMLTrackElement)
For the time being, there are many null-ness checks for both 'window' and
'document', and no mechanism to report errors other than returning nullptr
where possible. This will be addressed in a followup bug.
Also adds a test for mHead nullness in TextTrackCue::ConvertNodeTreeToDOMTree
to prevent SIGSEV.
