Bug 883834 - Touch events position is wrong on desktop when a DOM element is scrollable

I'm not an expert of Touch events, but this seems to fix this issue.
It's not very clear why we should pass `clientY` here, as the createTouch documentation mention a `PageY` argument:
https://developer.mozilla.org/en-US/docs/Web/API/DocumentTouch.createTouch
And `PageY` is described as:
`"The Y coordinate of the touch point relative to the viewport, including any scroll offset."`

I do not exactly understand what "including any scroll offset" means...
But the first part of the sentence seems to describe more `ClientY` than `PageY`!

Then I'm wondering if we have similar bugs for faked mouse and contextmenu events.