Monitors the window for changes to the ratio between device and screen
pixels, e.g. when the user moves the window from a high density screen to a
screen with normal density. Dispatches
goog.labs.style.PixelDensityMonitor.EventType.CHANGE events when the density
changes between the two predefined values NORMAL and HIGH.
This class uses the window.devicePixelRatio value which is supported in
WebKit and FF18. If the value does not exist, it will always return a
NORMAL density. It requires support for MediaQueryList to detect changes to
the devicePixelRatio.