I can't actually spot in the spec where it explicitly says to pass this
through (unlike the AudioContext constructor) - but clearly this needs
to be passed through for an OfflineAudioContext to actually have a
sample rate!
Which currently will always throw an exception as it is unimplemented
under the hood - but this gives all of the plumbing we need in order to
create a oscillator node as used in the reduced turnstyle testcase.
The spec doesn't tell us the exact value to use, but a minumum & maximum
range of supported values. Just to be consistent with another browser,
we follow the values that firefox appears to support from testing the
interface on my machine.
This function will be used in the AudioBuffer constructor, but is
defined in the spec as part of BaseAudioContext.
