libopus: Allow to enable custom modes.

The reason I'm not making this the default is because it seems to add
complexity and degrades performance of the library. For details have a
look at this lengthy discussion at:

https://bugs.debian.org/686777

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
fixup! chromium: Update all channels to latest versions.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>