Update description of 'disable app list' chrome flag.

This description was from a time when the app launcher was actually
disabled by this flag. Now it just resets the install state.

BUG=298849

Review URL: https://codereview.chromium.org/27000002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230100 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230100