modulate: move webaudio code out of constructor
modulate: move webaudio code out of constructor

Not all browsers support webaudio.  So move the webaudio initialization
code out of the constructor and into the modulateStream() function.

Signed-off-by: Sean Cross <d7e19930cc1f42c2d0781f4d9e6f1fe5891bf9cf@xobs.io>

chibitronics/ltc-webview-layer,chibitronics/ltc-webview-layer,chibitronics/ltc-webview-layer,chibitronics/ltc-webview-layer