progress-bar: indeterminate state not correctly exposed to screen readers

[mfairchild365]

What is the expected behavior?

The indeterminate state of a progress bar must be conveyed to users that are using screen reader software.

What is the current behavior?

Users hear something like 'progress bar, 50%'. This does not convey an indeterminate state.

What are the steps to reproduce?

1. Using VoiceOver for IOS (12.1.3) + Safari
2. Open the progress bar example
3. swipe right until you reach the 'indeterminate' radio button
4. double tap the screen to select the 'indeterminate' radio button
5. swipe right until you reach the progress bar
6. Observe that the progress bar is announced as 'progress bar, 50%'.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

• Angular Material Example 7.2.2 located at https://material.angular.io/components/
• NVDA 2018.4.1 + FireFox 64
• VoiceOver for IOS (12.1.3) + Safari (12.1.3)

Is there anything else we should know?

The ARIA spec states that authors should omit the aria-valuenow attribute to convey the indeterminate state. The attribute is set to aria-valuenow="50" after switching to the indeterminate in the example.

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