AudioBufferSourceNode.gain

WebKit and Blink implement a gain attribute on the AudioBufferSourceNode,
and looking at the implementation, it works exactly as if the output of
AudioBufferSourceNode was directly fed into a GainNode.  However, this
attribute is not spec'ed, and therefore Gecko doesn't implement it.

Over in https://bugzilla.mozilla.org/show_bug.cgi?id=861591, we have a game
which is relying on AudioBufferSourceNode.gain and breaks in Firefox
Nightly because that attribute does not exist.  We can do either of the
following two thing:

1. The right thing to do: continue to not spec AudioBufferSourceNode.gain
and remove it from both WebKit and Blink.  I doubt that there is a lot of
content out there using this attribute, given how late this bug was found
in Gecko.

2. If WebKit/Blink is not willing to remove this non-standard attribute,
then we should perhaps spec it.  This will be sad, since this attribute
doesn't really serve any purpose and it's unusual for a node to support the
functionality of another node in this way.

Regardless of the outcome of this discussion, I will try to reach out to
the author of that game and ask them to change their code based on the Web
Audio spec.

Thanks!
--
Ehsan
<http://ehsanakhgari.org/>