Ctrl+TAB and Ctrl+Shift+TAB?

It's useful that Ctrl+TAB is the shortcut for moving to the next tab to the right, but strangely, I can't find that shortcut in the shortcuts list. I want to add Ctrl+Shift+TAB for moving to the next tab to the left (as it is in the Chrome browser, for example), but I can't find that command anywhere in the shortcuts list either. Halp!

Solomon