Small wrapper around clearButtonEl.isVisible() to handle setVisible animation that may still be in progress.
function() {
            if (this.animateClearButton && this.animateWithCss3) {
                return this.clearButtonEl.hasCls(this.clearButtonCls + '-on');
            }

            // This should not be necessary (see Element.setVisible/isVisible), but else there is confusion about visibility
            // when moving the mouse out and _quickly_ over then input again.
            var cachedVisible = Ext.core.Element.data(this.clearButtonEl.dom, 'isVisible');
            if (typeof(cachedVisible) == 'boolean') {
                return cachedVisible;
            }
            return this.clearButtonEl.isVisible();
        }