Don't enumerate unmapped top-level windows.  This occurs most often in the Compiz window manager when the browser window is moved partially off-screen.
Don't enumerate unmapped top-level windows.  This occurs most often in the Compiz window manager when the browser window is moved partially off-screen.

BUG=15023
TEST=Extensive tab dragging in Compiz window manager, including a browser window that is partially off-screen.
Review URL: http://codereview.chromium.org/146059

git-svn-id: http://src.chromium.org/svn/trunk/src@19083 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: ed3bf9ec19ab4e02174827e29acdc5fa433c7db4
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser