Emulate support of touch events

[sadym-chromium]

Touch events specification defines Web API for touch events, including observable GlobalEventHandlers, allowing web application to identify if the user agent provides touch input.

We propose to add a command emulation.setTouchEventsEnabled, analog of CDP Emulation.setTouchEmulationEnabled with the following interface:

emulation.SetTouchEmulationEnabled = (
  method: "emulation.setTouchEmulationEnabled",
  params: emulation.SetTouchEmulationEnabledParameters
)

emulation.SetTouchEmulationEnabledParameters = {
  enabled: bool,
  ? maxTouchPoints: js-uint
}

[lutien]

Sounds good to me, I think I would prefer setTouchEmulationEnabled over setTouchEventsEnabled to indicate that we really enable or disable only the emulation of the events and not the events themselves. In case disabling of the events doesn't work for e.g. on mobile.