Added UseCounters for touchstart, mousedown, pointerdown invocation

Also added a PointerEventDispatch counter specific to pointerdowns,
so be able to guess the # of JS-fired events (the difference between
PointerEventDispatchPointerDown & PointerDownFired).

BUG=
TBR=isherman@chromium.org

Review-Url: https://codereview.chromium.org/2160353002
Cr-Commit-Position: refs/heads/master@{#406487}
4 files changed