column-count property doesn't work on Safari

## What's the issue?

In iOS Safari, `column-count` property doesn't work as intended because of the conflict with the following css setting. It's working as expected on Google Chrome.

`.i-amphtml-layout-size-defined {overflow: hidden!important;}`

### Screenshot
#### Google Chrome

#### Safari

## How do we reproduce the issue?
* https://shoho.github.io/column-count-test/index.html

## What browsers are affected?

* Safari only

## Which AMP version is affected?

2005151844001

Start with the reproduction at https://shoho.github.io/column-count-test/index.html and compare the Safari and Chrome layouts. Inspect the `.i-amphtml-layout-size-defined { overflow: hidden!important; }` setting and determine a change that restores the intended `column-count` behavior in iOS Safari. Done means the reproduction works in Safari without regressing Chrome.

Written by the indexing model from the issue text.