Steps to reproduce (assumes ChromeVox or similar)

1. Open https://elastic.github.io/eui/#/layout/header in a new tab.
2. Tab through to the header buttons shown in the picture

Actual Result

The screen reader simply reads "button" or something equally nondescript.

Expected Result

The screen reader should read "Search Button", "Notifications: 3", and "menu button" or sometihng similar

Category: #726: Elastic UI Header Accessibility
Relevant WCAG Criteria: 3.3.2 Input Assistance: Labels or Instructions