ScriptProcessorNode.channelCountMode should be explicit by default

AudioContext.createScriptProcessor() accepts the number of input channels
as an argument, but the channelCountMode is currently set to "max" by
default, which renders that argument moot by default.  I believe that we
need to change the default value of channelCountMode for
ScriptProcessorNode to "explicit" in order for this all to make sense
together.

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