app_list: Use existing item when installing an already installed app.

BUG=306004

Review URL: https://codereview.chromium.org/26901004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228124 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228124