Remove redundant Mac event structures

The scrollCount field is unused.

The unacceleratedScrollingDeltaX/Y field always equal deltaX/Y (by
inspection, then by adding an assert and testing), so use those
directly.

BUG=133097

Review URL: https://codereview.chromium.org/658953003

git-svn-id: svn://svn.chromium.org/blink/trunk@183851 bbb929c8-8fbe-4397-9dbb-9b2b20218538