That helps to pass WPT tests
under /webaudio/the-audio-api/the-gainnode-interface/ctor-gain.html
(cherry picked from commit 4998385c7aeae21939f4ea24bb4f8c6eb477840b)
As with all other current audio nodes we still need to wire up the
inputs and outputs so it can be properly used in an audio context - but
this is enough to implement the public IDL interface.
