Bug 836076 - Part 1: Move AudioEventTimeline.h to content/media; r=roc

--HG--
rename : content/media/webaudio/AudioEventTimeline.h => content/media/AudioEventTimeline.h
Bug 836076 - Part 2: Make it possible to send AudioEventTimeline objects as commands to the MSG thread; r=roc
Bug 836076 - Part 3: Make AudioParams call back into their owning node when they're modified; r=roc

We need this in order to update the MediaStreamGraph thread when an
AudioParam changes.  This enables each AudioParam to be registered with
a callback from its owner node, so that the owner node can have custom
processing code for each AudioParam's mutation.