Bug 1015783 - Add a devtools API for Web Audio; r=padenot,smaug

See bug 980506 for an extensive discussion about this.  This patch adds
three APIs to AudioNode in order for us to be able to build awesome
devtools on top of it.

* Weak reference API.
  This patch allows one to hold a weak reference to all AudioNode's
  using Components.utils.getWeakReference().  That way, the devtool's
  inspection code would not change the lifetime of AudioNodes.
* AudioNode.id
  This is a chrome-only unique and monotonically incrementing ID for
  AudioNode objects.  It is supposed to be used in order for the
  devtools to be able to identify a node without having to keep it
  alive.
* webaudio-node-demise
  This is an observer notification that is called every time an
  AudioNode gets destroyed inside Gecko.  The ID of the corresponding
  node is passed to this notification.