On Windows, Linux, and Android we have multiple audio backends which are selected dynamically based on available functionality. It's important for support purposes to know which backend we're using, and it's often difficult to explain how the end user may derive that information from their system. Adding the backend to about:support makes this much easier. This is the bare minimum I wanted in bug 1197045; I'll leave that bug to cover adding additional information such as the audio hardware defaults.

This is based on the DVXA2 changes in bug 1151611. We don't have a dedicated media section, to I'm adding it beside the only existing media playback related about:support entry. It'll probably make sense to expand into our own media section in bug 1197045. r? felipc since he reviewed the DXVA2 additions.

Pushed by mgregan@mozilla.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/b558126227ac Add libcubeb backend to about:support. r=felipc