Add contents scale change event (handles NPNVcontentsScaleFactor).

This commit adds a new event, called ContentsScaleFactorChangedEvent, raised when browser notifies the plugin of dpi change.

I made this Mac only since this is most important for Retina displays (and that was the only system I was able to test my change).

Please let me know what you think, and especially if you wanted me to change any of it.

Thanks
Grzegorz Milos (gregor.milos@bromium.com)