I checked it appears used in filesystem and webrtc it looks like. Not sure who wants to own it.

https://w3c.github.io/webrtc-pc/#legacy-interface-extensions uses VoidFunction and https://www.w3.org/TR/webdatabase/#asynchronous-database-api has a SQLVoidCallback type.

https://w3c.github.io/filesystem-api/#directory-webidl has a promise-based API.

If there's no usage left in any spec, we should instead transition to other types. Renaming to reflect that.

Affected IDL files:
Source/modules/filesystem/DirectoryEntry.idl
Source/modules/filesystem/Entry.idl
Source/modules/peerconnection/RTCPeerConnection.idl
Source/modules/webdatabase/Database.idl

https://w3c.github.io/filesystem-api/#directory-webidl is not implemented by anyone, let alone Chrome. The relevant snapshot of the earlier spec we implement in Chrome is:

https://dev.w3.org/2009/dap/file-system/file-dir-sys.html#the-voidcallback-interface

We can make up a FileSystemVoidCallback if we want.

https://heycam.github.io/webidl/#VoidFunction might be a suitable type for most or all of these cases.

VoidFunction sgtm

(If anyone is depending on 'callback interface' behavior for these I'll eat my hat. Well, no, because it's a nice hat. I'll eat someone's hat.)

Funny enough, lots of things that look like callback interfaces don't actually have that behavior, only EventListener and NodeFilter do, see  issue 630986  and  issue 658073 . So the chance of hat for dinner is very, very low.

And FYI, both the WebSQL and FileSystem ones were decorated with an obsolete extended attribute implying this behavior anyway: 

[Callback=FunctionOnly, NoInterfaceObject]
interface SQLVoidCallback {
  void handleEvent();
};

[Callback=FunctionOnly]
interface VoidCallback {
    void handleEvent ();
};

(in the spec I mean)

That's funny, I guess there was a time when callback functions didn't exist and that was the only way?

Making this non-trivial in the WebSQL case: InspectorDatabaseAgent subclasses VoidCallback, overriding handleEvent(). We don't currently allow the a generated VoidFunction::call() to be overridden.

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