Both chromium and chromium bin create and invalid bit of code when make_desktop_entry is called at the end of their ebuilds. Instead of specifying ${PN}-whatever.png, leave off the extension .png. This must be changed, otherwise the generated .desktop file is invalid and there's a good chance that the icon won't show up in your menu. Check it with desktop-file-validate: although eutils.eclass does this, it doesn't die, and instead only prints an error. This is a problem because on Xfce at least, 7/10 applications won't show up in the menu if their .desktop entries are at all invalid, especially their icon info. So just punt the .png from make_desktop_entry, and you'll be all set.
Sorry, they worked in my xfce desktop, so I did not see this validity problem Fixed for both chromium and chromium-bin!
