WheelEvent's deltaX/deltaY should report real amount of pixels scrolled

As per the specification, the DOM3 WheelEvent's deltaX/deltaY should report
the real amount of pixels scrolled. This is also the way IE and Firefox
behave.

However, Blink was reporting the number of mouse wheel ticks multiplied
by a multiplier (120) which does not necessarily match the real amount
of pixels that is scrolled.

R=arv
BUG=227454

Review URL: https://chromiumcodereview.appspot.com/23034006

git-svn-id: svn://svn.chromium.org/blink/trunk@156603 bbb929c8-8fbe-4397-9dbb-9b2b20218538