WheelEvent created using document.createEvent() is not properly initialized

WheelEvent created using document.createEvent() is not properly initialized,
its deltaX, deltaY, deltaZ attributes are uninitialized. The specification
indicates that those should be initialized to 0.

R=arv
BUG=227454

Review URL: https://chromiumcodereview.appspot.com/23381002

git-svn-id: svn://svn.chromium.org/blink/trunk@156500 bbb929c8-8fbe-4397-9dbb-9b2b20218538