Bug 916773 - Initialize mLatency in SharedBuffer's constructor. r=ehsan
b=916422 make OfflineAudioContext::StartRendering() throw if called more than once r=ehsan

--HG--
extra : transplant_source : %B6T%C0C%F0/%03%A3C%25%9B%EF%A8%C41%8E%86%E9%FCr
b=910171 add a general means to keep active nodes alive from the AudioContext r=ehsan

AudioNodes that keep playing or tail-time references need to have these
references cleared when an AudioContext has completed or is shut down by the
window.

Storing references on the AudioContext instead of on the AudioNodes will allow
the AudioContext to report playing references to the cycle collector until
offline rendering starts for bug 914033.  This is not necessary for tail-time
references, but it is tidier to use the same code for playing and tail-time
references.

--HG--
extra : transplant_source : %E0%F1%06%BFV%B6XI%9BX%8E%8D7%3FsU%8F%F9%14r
b=910171 register tail references with the AudioContext so they can be removed when the rendering is complete r=ehsan

--HG--
extra : transplant_source : %8C%C3%C2%2B%1B%E9Q%9B%0E%DE%B4%AA%02%B2%D1%A0S%F1%EC%0D
b=916680 don't keep active AudioNodes alive from an AudioContext that has shut down r=ehsan

--HG--
extra : transplant_source : %20%3Fl%E0%D7R%7B_%E6%B0%C7%BB%3A%E7%21%9D%F1%A2%F4%1C