GitLab

Add two types of messages to announce to screen reader/Chromevox when
a file or multiple files are open.

Add a new 0px-wide DIV to not be shown in the UI but still be accepted
by screen reader, if we set display:none screen reader ignores it. This
DIV has aria-live="polite" which makes screen reader read it whenever
its content is updated, the "polite" part means that it will wait for
any current content being read before announcing this text.

Add a new method on FileManagerUI |speakA11yMesage| to change the
content of the DIV above so the text is announced via screen reader.
When IN_TEST, this also stores the text so it can be checked via in the
tests. Add a test helper function to return all messages announced via
this new method.

Add a check for the A11y message on Open Image test.

Test: browser_test --gtest_filter="OpenImageFiles/FilesAppBrowserTest.Test/imageOpenGalleryOpenDownloads"
Bug: 428530
Tbr: aboxhall@chromium.org
Change-Id: I713300a828912722c4fbe795f0505ed7db371fe0
Reviewed-on: https://chromium-review.googlesource.com/c/1347956
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610522}