Fix movementX/Y attributes for touch pointer events

Keep track of previous coordinates of touch points and calculate the deltas
and store them inside the WebTouchPoints. Since pointerevents have
movementX/Y attributes which they expose to js this CL calculates the
correct values for movementX/Y of touch pointerevents.

BUG=678258

Review-Url: https://codereview.chromium.org/2624783002
Cr-Commit-Position: refs/heads/master@{#449714}