Address API council feedback for setOffscreenPreRaster.

     * offscreen but attached to a window. Turning this on can avoid

     * rendering artifacts when animating an offscreen WebView on-screen.

     * Offscreen WebViews in this mode use more memory. The default value is

     * false.

     * false.<br>

     * Please follow these guidelines to limit memory usage:

     * - WebView size should be not be larger than the device screen size.

     * - Limit use of this mode to a small number of WebViews. Use it for

     * <ul>

     * <li> WebView size should be not be larger than the device screen size.

     * <li> Limit use of this mode to a small number of WebViews. Use it for

     *   visible WebViews and WebViews about to be animated to visible.

     * </ul>

     */

    public abstract void setOffscreenPreRaster(boolean enabled);