Bug 998179 - Refactor how MediaStreamGraph get and use their sample rate. r=roc

Use the sample rate passed to the OfflineAudioContext constructor in
MediaStreamGraph::CreateOfflineInstance, and pass the preferred mixer sample
rate to the (real time) MediaStreamGraph constructor.

Then, always use this sample rate for the lifetime of the graph.

This patch needed to pass the sample rate to the AudioMixer class to avoid
relying on globals like it was done before.

--HG--
extra : rebase_source : 2802208819887605fe26a7040998fc328b3c9a57