[Merge] lp:~yuningdodo/ubuntu/trusty/unity-settings-daemon/unity-settings-daemon.lp1396464-set-volume-to-zero-before-mute into lp:ubuntu/trusty-updates/unity-settings-daemon

[Yu Ning]

Sebastien, thanks very much for the review, I have only tested it on my own trusty box, however I checked the trunk source code just now and I think the patch could be safely applied to trunk.
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https://code.launchpad.net/~yuningdodo/ubuntu/trusty/unity-settings-daemon/unity-settings-daemon.lp1396464-set-volume-to-zero-before-mute/+merge/242869
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