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

git-svn-id: svn://svn.chromium.org/blink/trunk@159223 bbb929c8-8fbe-4397-9dbb-9b2b20218538