Replace MouseWheel events with GestureScrollBegin in WebContentsObserver::DidGetUserInteraction
crbug.com/568183 implements GestureScrollBegin for mouse wheel events.
Once this has landed, the mouse event coalescing currently performed by
WebContentsObserver::DidGetUserInteraction will be redundant.
This CL removes the mouse event coalescing, and changes
DidGetUserInteraction to fire on GestureScrollBegin events. The event
type used to signal scrolls in the method is changed from MouseWheel to
GestureScrollBegin, as scroll events will now be fired on mobile scrolls
as well as wheel scrolls. Finally, in clients of DidGetUserInteraction,
MouseWheel is replaced with GestureScrollBegin, and references to
'wheel' are replaced with 'scroll'.