www/firefox: Fix for jitter when using the cubeb OSS audio backend.

Cherry-pick the following changes from Mozilla's cubeb.git:
b690782c19728ee7a5675237b0b30249f4e1bf5e
8da61ef32d386ffd5f00f025578d4429523ca45b
1099a3007817fa34c1f91e4710ec046021147bed

The need for this patch will eventually go away.

Discussed with: jbeich@ and khng@
Obtained from: https://github.com/mozilla/cubeb.git
Approved by: pi (implicit)