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This change just exposes information about what tabs are selected
and fires proper notifications when tab selection changes. This will
allow screen readers to announce when switching tabs even if the
web content doesn't have focus. It's also a good first step towards
making the tab strip fully accessible too (e.g. making it possible to
perform operations on multiple tabs with just the keyboard).

As part of this change I renamed the "selection changed" event to
"text selection changed" so that there's no confusion between the
text-selection-related events, and tab/list selection events.

BUG=100412

Review URL: https://codereview.chromium.org/349033010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282396 0039d316-1c4b-4281-b951-d872f2087c98