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Add a message to screen reader once a volume has been ejected, it uses
|speakA11yMessage| function introduced on crrev.com/c/1347956.

--gtest_filter="Providers/FilesAppBrowserTest.Test/providerEject"

Test: browser_test
Bug: 758638
Change-Id: I16e20f828c05368a41046d71fcfbcf04cfb212f4
Reviewed-on: https://chromium-review.googlesource.com/c/1348880Reviewed-by: Anand Mistry <amistry@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610526}