修复miox-css上webview:display none的冲突

[wufengjie]

解决问题：

• 解决webview回退时动画缺失

更改点：

miox-animation中增加样式：

.mx-webview{
    &.active,
    &.inactive{
        display: block;
        visibility: visible;
    }
}

[coveralls]

Coverage remained the same at 90.03% when pulling c9ba658 on wufengjie:animationfix into 75aaa7e on 51nb:master.