ScriptProcessorNode bufferSize should be optional

We've discussed before how, if the specified buffer size matters in terms
of avoiding underruns etc, it's likely that most Web authors will not be
able to choose a buffer size that's appropriate for the Web Audio
implementation and underlying hardware.

So, I think we should make the bufferSize parameter optional. When it's not
given, the UA will choose a suitable buffer size, which will be constant
for the lifetime of that ScriptprocessorNode. We would encourage authors to
not specify a fixed buffer size.

We'll do this in Firefox if there are no objections.

Rob
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