Bug 877039 - Set the Web Audio maximum channel count to 32; r=roc

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extra : rebase_source : 7e5bb51cd3bd9b4fbe01649af8f14e17cf6b3042
Bug 865806 - Part 1: Continue walking up the frame tree when hitting a transparent frame as we're trying to determine whether a given element's background is dark or not; r=roc

This heuristic improvement gives us correct dark background detection in
pdf.js, among other cases.

--HG--
extra : rebase_source : 5a8f920da1b63f6452ec709745b2154b6fc17ceb