Paint Flashing in Firefox Nightly

Yesterday I landed a new feature in the Firefox DevTools: paint flashing. What it does is to highlight areas being painted by Gecko. Avoiding useless painting operations can help you to build more performant web pages.

Note for Firefox developers: The pref nglayout.debug.paint_flashing still works, but I encourage you to try the paintflashing command: If you turn on the pref devtools.chrome.enabled, you can type the command (shift-F2 to bring the command line) paintflashing on --chrome to enable paint flashing only for the chrome (the --chrome option is not available if you don't turn on this pref).
I introduced a new property to nsIDOMWindowUtils: paintFlashing. Setting it to true will turn on paint flashing for the specified window, and to false will clear the colors and stop paint flashing.