Intent to implement and ship: InputEvent.data and InputEvent.dataTransfer

[Masayuki Nakano]

We need to implement InputEvent.data and InputEvent.dataTransfer because
they are important information for web apps especially for "beforeinput"
event listeners. So, we need to implement these attributes before
implementing "beforeinput" event. The bug is here:
https://bugzilla.mozilla.org/show_bug.cgi?id=998941

Those attributes tell what content is inserted ("input") or will be
inserted ("beforeinput") to web apps.

The former is declared by UI Events:
https://w3c.github.io/uievents/#dom-inputevent-data

The latter is declared by Input Events:
https://rawgit.com/w3c/input-events/v1/index.html#dom-inputevent-datatransfer
https://w3c.github.io/input-events/#dom-inputevent-datatransfer

And their values are defined by Input Events:
https://rawgit.com/w3c/input-events/v1/index.html#overview
https://w3c.github.io/input-events/#overview

Those are implemented by both Chrome and Safari, but their
implementation are not enough though (e.g., "input" event is not set
those values properly in a lot of cases). But I'm going to add check of
those attributes into WPT as far as possible at landing the patches and
I'll file bugs for them.

Currently, I plan to enable them by default in all channels like
InputEvent.inputType. If we would get web-compat issues, e.g., they are
used to detect "beforeinput" event support, then, I'd disable them in a
follow up bug.

--
Masayuki Nakano <masa...@d-toybox.com>
Software Engineer, Mozilla