table component, header is notched on chrome72 and without vertical scrollbars

### Element UI version
2.15.8

### OS/Browsers version
windows

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/youpinyao/pen/yLvBXMJ

### Steps to reproduce
open in windows and chrome version 72

### What is Expected?
table component, header is notched on chrome version 72 and without vertical scrollbars

### What is actually happening?
table component, header is full on chrome version 72 and without vertical scrollbars