Remove [NoInterfaceObject] from several WebAudio IDL interfaces

Source/modules/webaudio/GainNode.idl

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 [
-    NoInterfaceObject,
     Conditional=WEB_AUDIO
 ] interface GainNode : AudioNode {
     // FIXME: eventually it will be interesting to remove the readonly restriction, but need to properly deal with thread safety here.
     readonly attribute AudioParam gain;
 };