User agents are apparently now expected to do this; failure to do so
results in the video file data transfer getting throttled down to rates
such as 80 kB/s, 60 kB/s or 40 kB/s, below playback rate, and usually
resulting in a video that hangs upon loading or every few seconds, and
is impossible to play. This behavior seems to have first appeared in
June, but been fully rolled out only last week.

Just like with URL signatures, we interoperate with YouTube by
fulfilling what's apparently expected from us, using the same approach
as so far: we parse the descrambling rules from the javascript code, and
apply them.

Fixes #26174