[Bug Report] Using el-table in electronic-vue, it can't render properly.

### Element UI version
2.9.1

### OS/Browsers version
macOs Mojave 10.14.5

### Vue version
2.6.10

### Reproduction Link
https://github.com/Char-set/element-ui-issue-in-electron-vue.git

### Steps to reproduce
1、拉取项目代码
2、yarn
3、yarn run dev

### What is Expected?
el-table渲染正常

组件文件在 src -> renderer -> components -> LandingPage.vue

### What is actually happening?
el-table 渲染失败

在devtools中查看 el-table-header 组件属性 发现：

conputed属性中 columns 数组为空，但是columnsCount 属性值确为 3