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Since we don't have alt text yet, accessibility screen readers have
nothing useful to say in screen readers.  Until alt text is available,
disable the images from appearing in the screen reader.

This is done by removing the accessibility child object from a parent.
In this case, we introduce a new fake parent object for the small icon,
and make the existing parent ignore the image for the large icon and
the large image of an image notification.

BUG=381363

Review URL: https://codereview.chromium.org/325203003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@277957 0039d316-1c4b-4281-b951-d872f2087c98