SnackbarContent role="alertdialog" is incorrect role depending on Snackbar usage

[emilyuhde]

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

When using a Snackbar, the SnackbarContent adds role="alertdialog". According to MDN, if an alert includes interactive controls, the role should be set to "alertdialog". If it does not, then it should be set to "alert".

If the Snackbar is given some interactive buttons then role="alertdialog" is appropriate, but since users can decide to only set text on Snackbars, they should be able to customize which role they would like to have set to meet the specification.

"If an alert also provides interactive controls (such as form controls that allow the user to rectify a problem, or an "OK" button that discards the alert) the alertdialog role should be used instead."
https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Techniques/Using_the_alert_role

Expected Behavior 🤔

The user should be able to set their own role attribute by passing in a prop, just like how the message can be set.

Steps to Reproduce 🕹

1. Go to the Material UI site's Snackbars demo page https://material-ui.com/components/snackbars/
2. Open/view any of the Snackbars that do not have buttons, such as the "Positioned snackbars" section's "TOP-CENTER" Snackbar.
3. Right click and inspect the Snackbar.

You will notice that the Snackbar's div shows role="alertdialog" even though there are not interactive elements on the Snackbar. This is being hardcoded on line 55 in SnackbarContent.js: https://github.com/mui-org/material-ui/blob/824294842997c79494b4f328bb7903774a05422b/packages/material-ui/src/SnackbarContent/SnackbarContent.js#L55

Context 🔦

This is an accessibility concern as the role helps screen readers to read out alert messages as they pop up onto screens and incorrect roles can be confusing to screen reader users.

Your Environment 🌎

Tech	Version
Material-UI	v4.3.1
React	16.8.0
Browser	all
TypeScript	N/A
etc.

[emilyuhde]

I'm working on a fix for this.