Revert 159223 "Cleanup: Add more conversion helpers for usage."

The ASSERTs are triggers on some fast/dom/DeviceMotion and
fast/dom/DeviceOrientation/ tests.

> Cleanup: Add more conversion helpers for usage.
> 
> Adding toDeviceMotionEvent and toDeviceOrientationEvent conversion
> functions to replace static_cast, and update toWheelEvent and toGestureEvent
> so that they accept NULL argument for consistency.
> 
> R=tkent, jamesr
> BUG=None
> TEST=None; No behavior changes;
> 
> Review URL: https://codereview.chromium.org/26434002

TBR=r.kasibhatla@samsung.com

Review URL: https://codereview.chromium.org/26541009

git-svn-id: svn://svn.chromium.org/blink/trunk@159256 bbb929c8-8fbe-4397-9dbb-9b2b20218538