[bug report] cascade, size = small, filterable. When the page is enlarged to 110%, the height change frequently triggers the resizeobserver

### Element UI version
2.15.0

### OS/Browsers version
mac/chrome

### Vue version
2.5.17

### Reproduction Link
https://codepen.io/wanzizi/pen/ExZLXEm

### Steps to reproduce
1.
2.浏览器放大至110%

### What is Expected?
高度固定不变

### What is actually happening?
级联框高度一直在33px和34px之间不停变动，导致级联框抖动，断点调试是因为每次tags获取到的高度不一致，从而频繁触发ResizeObserver