fix: auto sized children break horizontal scroll

[eljefe223]

🐛 Bug Report

When children inside of horizontal-scroll have their widths set to auto the flippers will not appear.

💻 Repro or Code Sample

<fluent-horizontal-scroll>
     <fluent-card style="width:auto"></fluent-card>
     <fluent-card style="width:auto"></fluent-card>
     <fluent-card style="width:auto"></fluent-card>
     ...Etc...
</fluent-horizontal-scroll>  

🤔 Expected Behavior

😯 Current Behavior

💁 Possible Solution

🔦 Context

🌍 Your Environment

• OS & Device: [e.g. MacOS, iOS, Windows, Linux] on [iPhone 7, PC]
• Browser [e.g. Microsoft Edge, Google Chrome, Apple Safari, Mozilla FireFox]
• Version [e.g. 1.8.0]