Clarifying the accepted arguments to the constructor of OfflineAudioContext

Currently the spec doesn't talk much about what needs to happen when
invalid arguments are passed to this constructor.  For now I'm going to
disallow a length of zero which doesn't make sense (and crashes Chrome) in
Gecko, and we already throw with non-positive sample rates that fall
outside of the range which we can handle.  It also doesn't make sense to
pass 0 for the channel count, in my opinion.

I'm planning to modify Gecko to throw NOT_SUPPORTED_ERR for all of these
cases.  Not touching the spec for now since this is all going to change
soon.


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