<amp-call-tracking> doesn't work when is has 'display: contents'

## What's the issue?

I experienced this issue on a site where the amp-call-tracking element had its display property set to contents, and amp-call-tracking didn't change the number as it was supposed to.

## How do we reproduce the issue?

I've made a codepen with this exact issue reproduced.
https://codepen.io/sunturion/pen/qBaYWVX

in the first instance, the amp-call-tracking element is `display: block`
and in the second instance it is set to `display: contents` and as you can see, it doesn't work.

## What browsers are affected?

This is the same on all supported browsers

## Which AMP version is affected?

I havn't tested this in older versions of AMP, or noticed this issue before.