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This will specifically allow screen readers to query the accessible
object for the main window and quickly get a list of an alerts that
are visible - for example infobars, bubbles, and the Chrome menu itself
if it has an alert icon in it.

BUG=369903

Review URL: https://codereview.chromium.org/266963002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270074 0039d316-1c4b-4281-b951-d872f2087c98