Bug 891773 - Do not assume RelocatablePtr is only used on JSObject; r=billm

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extra : rebase_source : ff327374ac7900626fe166b307fba5b1fcd31f3f
Bug 891773 - Check ArrayBufferView's back reference before marking; r=sfink

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extra : rebase_source : 9509d8304020457f31cbb6b90ae5f6f4cc9270db
Bug 790559 - Part 1: Apply DownmixToStereo function when decode multichannel Opus and Vorbis. r=rillian
Bug 790559 - Part 2: Correct test_framebuffer.html mochitest to expect two channels. r=rillian