Make sure that ol.MapBrowserEvent#getPixel() always returns a valid value

Because of a bug in closure-library [1], `ol.MapBrowserEvent#getPixel()` crash when the event type is `touchend` or `touchcancel`. This PR adds a workaround by computing the `pixel_` attribute in the constructor.

[1] https://code.google.com/p/closure-library/issues/detail?id=588