Emacs doesn't have any keybinding to these functions by default ... nor menu-items, nor anything else. You might find them helpful when working with text formatting languages like Markdown or reST.
(global-set-key "\C-c>" 'increase-left-margin)
(global-set-key "\C-c<" 'decrease-left-margin)
(global-set-key "\C-c\C-r" 'increase-left-margin)
(global-set-key "\C-c\C-l" 'decrease-left-margin)
These keybindings come from emacs's python-mode. Note that the prefix key C-c is "reserved for users" -- nevertheless, python-mode sets 28 keybindings beginning with it.