Bug 875144 - Disallow creating AudioBuffers of 0 length; r=roc
Bug 870179 - Fix privacy leak in BackgroundPageThumbs when private browsing is active. r=ttaubert
Bug 875529 - Correctly reserve some extra junk at the end of the down-mixing buffer; r=roc

CUSTOM_CHANNEL_LAYOUTS is 6, and we use the index 6 to denote channels that
must be ignored, therefore we need an array of 7 values here.