Add FaviconWebUIHandler to AppLauncherPageUI

The chrome://apps page needs to be able to send getAppIconDominantColor.
This was causing a DCHECK in debug builds.

BUG=327967

Review URL: https://codereview.chromium.org/113683002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240618 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240618