Bug 849918 - Initial support for PannerNode's 3D positional audio (equalpower panning model). r=ehsan,roc

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extra : rebase_source : d729da50f97b94ea0434a93ad481ac35954bfa9a
Bug 853360 - Implement the coneGain part of the AudioPannerNode. r=roc

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extra : rebase_source : c3a27ef2da8a080b9b310af14cf85f68f13df9a6