It looks like this is happening because the third-party software source for Google Chrome was disabled when you upgrade from one Ubuntu release to another. Then the solution to this problem is to re-enable the software source, making sure it is configured for the Ubuntu release you are currently running, and then update again.
To do this, open the Update Manager. Click
Settings... on the lower-left corner of the Update Manager window, to bring up the Software Sources window. Click the Other Software tab. Find the entries for Google Chrome (there will probably be two of them). They might say
disabled on upgrade to _____ with
_____ replaced by the codename of some Ubuntu release (e.g.,
precise). But they will not necessarily say that.
For each one, click on it and click
Distribution text box contains
stable, leave it that way. This is how the Google Chrome software source currently works.
If you are doing this for a future versions of Chrome which might work differently, or for some other software, make sure the
Distribution text box contains the codename (in all lower-case letters) of the Ubuntu release that you are currently running.
If you're not sure what that is, open a Terminal window (Ctrl+Alt+T) and run the command
lsb_release -c. That will tell you.
Thanks to Samik for pointing out that the current version of Google Chrome's software source does not use an Ubuntu release codename, but instead uses the general
stable codename from Debian.
Comment text box contains just
disabled on upgrade to _____, then you can go ahead and delete that text too.
OK. Make sure to do this for both Google Chrome software source entries.
Enable these software sources by clicking the empty checkbox next to them.
Close to quit out of the Software Sources window. You should now be able to update successfully.
(If this doesn't work, please leave a comment to facilitate further troubleshooting.)