DotCom's admin masterbar uses absolute position instead of fixed in small viewports

### Impacted plugin

mu wpcom

### Quick summary

When viewing a Woo Express site's wp-admin on a small screen (max-width 480px), the masterbar is positioned absolute instead of fixed.

Here's a screenshot: 

### Steps to reproduce

1. Open a Woo Express site
2. Navigate to wp-admin
3. Use a 460px viewport
4. Scroll and notice the issue

### A clear and concise description of what you expected to happen.

The masterbar should be fixed on top.

### Impact

Happens in the wp-admin context.

### Platform (Simple and/or Atomic)

Atomic since it only happens on wp-admin.

Reproduce the issue in wp-admin at a 460px viewport and inspect the masterbar styles used below 480px. Trace where the responsive positioning rule is defined. Done means the masterbar remains fixed at the top while scrolling in the stated Woo Express Atomic context.

Written by the indexing model from the issue text.