This code adds a function to a WindowObserver that notfies it of a window title
change, allowing to update the label in overview mode dynamically.
BUG=387130
TEST=WindowSelectorTest.CreateLabelUnderWindow
Review URL: https://codereview.chromium.org/331643004
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@279887 0039d316-1c4b-4281-b951-d872f2087c98