MouseEvent Cleanup: Replace isSimulated with new SyntheticEventType value.

SimulatedMouseEvents are currently used to represent both
synthetic mouse events without a location and synthetic
mouse events with a location. This is very confusing since
the second class of SimulatedMouseEvents behave exactly as
real mouse events (RealOrIndistinguishable).

We have cleaned up this mess by breaking the concepts apart
and replacing the first class of SimulatedMouseEvents with
Positionless mouse events, and reusing real mouse events for
the second class of SimulatedMouseEvents.

BUG=405990

Review URL: https://codereview.chromium.org/518723003

git-svn-id: svn://svn.chromium.org/blink/trunk@181359 bbb929c8-8fbe-4397-9dbb-9b2b20218538