Sound won't work, but at least tests won't fail

This will fix 1059258 and maybe other intermittents, most of the time on Windows 7 (not 8, not vista, 7 only, because of what seems to be a windows bug). This brings back the behaviour of the pre-refactoring MSG, in case of cubeb_stream_init failure: sound will not be output, but everything else will continue working (AnalyserNode, ScriptProcessorNode, video, etc.). This is absolutely edge-casey (apparently it can be reproed when creating 200 <audio> and trying to create an AudioContext right after on certain version of windows 7). Also, tests will be shown as green, because there is little we can do against os bugs.

Comment on attachment 8487160 [details] [diff] [review] When cubeb_stream_init fails, fallback to using a SystemClockDriver. r= Approval Request Comment [Feature/regressing bug #]: Bug 848954 [User impact if declined]: Intermittents on Windows; lockups/failures in odd timing edgecases [Describe test coverage new/current, TBPL]: Covered which is why we see the intermittent. On inbound. [Risks and why]: low risk - we don't want to simply fail to start a driver when we need one. [String/UUID change made/needed]: none

Comment on attachment 8487160 [details] [diff] [review] When cubeb_stream_init fails, fallback to using a SystemClockDriver. r= Aurora+