Bug 951770 - Reject invalid Opus pre-skip in WebM. r=doublec

The 'pre-skip' value representing encoder delay in the Opus
format is recorded once in the .opus representation, but for
WebM it's in the file both in the CodecDelay element and in
the CodecPrivate data. We now reject files where these two
fields don't match.

Our detodos.webm file was exactly this sort of mismatched
file. It has been renamed and added to the invalid file list
to verify we now reject it. A new detodos.webm replaces it,
remuxed from detodos.opus with a bugfixed mkvmerge.

Based on a patch by Jan Gerber.

--HG--
rename : content/media/test/detodos.webm => content/media/test/invalid-preskip.webm