[touch-events] TouchEventInit should inherit from EventModifierInit (#40)

EventModifierInit is defined here: https://w3c.github.io/uievents/#idl-def-EventModifierInit

We should simply have TouchEventInit inherit from it and get rid of the alt, ctrl, meta, shift definitions in its dictionary in the Touch spec.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/touch-events/issues/40