[Bug Report] popper__arrow dissapears with fixed height and overflow: auto; for .el-dropdown-menu

### Element UI version
0.1.0

### OS/Browsers version
ubuntu/Chrome

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/anon/pen/MdmeGb?&editable=true

### Steps to reproduce
```css
.el-dropdown-menu.el-popper {
height: 105px;
overflow: auto;
}
```
This leads to disappearing of popper__arrow

### What is Expected?
popper__arrow should be same as w/o overflow: auto

### What is actually happening?
popper__arrow dissapears

Start with the linked CodePen reproduction and inspect the el-dropdown-menu/popper styles while applying the fixed height and overflow:auto rule. Compare the result with the same menu without overflow; done means the popper__arrow remains visible and matches the expected appearance.

Written by the indexing model from the issue text.