ResizeObserverEntry = function(target, rectInit) {
    var contentRect = createReadOnlyRect(rectInit);

    // According to the specification following properties are not writable
    // and are also not enumerable in the native implementation.
    //
    // Property accessors are not being used as they'd require to define a
    // private WeakMap storage which may cause memory leaks in browsers that
    // don't support this type of collections.
    defineConfigurable(this, { target: target, contentRect: contentRect });
}
Updates 'broadcastWidth' and 'broadcastHeight' properties with a data
from the corresponding properties of the last observed content rectangle.

@returns {DOMRectInit} Last observed content rectangle.