Copy the periodicWave data into a shareable buffer.
Add a SetBuffer call to send the buffer data to the
OscillatorNodeEngine. Call into blink's PeriodicWave
implementation to generate bandlimited tables bracketing
the desired frequency and interpolate the output data
from them.
Change the PeriodicWave constructor to only take one
length, since both arrays must be the same size.
Change OscillatorNode's SetType to throw INVALID_STATE
instead of NOT_IMPLEMENTED if js tries to assign
type = 'custom' directly.
