The wmctrl package offers access and manipulation of application windows and would do what you want. Install this little package first:
sudo apt-get install wmctrl
Here is the man page for wmctrl:
When you look at the command line switches, you will see that -a will raise and focus a window. In order to know which window to focus, make sure you have a Chromium window open, and then run
This will give you a list of all open windows and their titles. Chromium will most likely always have "Chromium" in the end of it's window title, so you can use that.
With this information at hand, you can construct your command for the shortcut. Open the Keyboard preferences, go to Shortcuts, click on Custom Shortcuts, click the + sign and enter:
Name: Focus Chromium Shortcut
Command: wmctrl -a 'Chromium'
I am using Unity and just tried this with Google Chrome, it did the trick. It should work in Gnome-Shell as well.
Pretty cool idea :)