Add strings for the canvas prompt patch.

These strings are used in one of our FF patches (see bug #6253), but we
include them here so they can be translated.

src/chrome/locale/en/torbutton.properties

@@ -43,3 +43,10 @@ torbutton.popup.device_full = The disk appears to be full. Please free up space
 torbutton.title.prompt_torbrowser = Important Torbutton Information
 torbutton.popup.prompt_torbrowser = Torbutton works differently now: you can't turn it off any more.\n\nWe made this change because it isn't safe to use Torbutton in a browser that's also used for non-Tor browsing. There were too many bugs there that we couldn't fix any other way.\n\nIf you want to keep using Firefox normally, you should uninstall Torbutton and download Tor Browser Bundle. The privacy properties of Tor Browser are also superior to those of normal Firefox, even when Firefox is used with Torbutton.\n\nTo remove Torbutton, go to Tools->Addons->Extensions and then click the Remove button next to Torbutton.
 torbutton.popup.short_torbrowser = Important Torbutton Information!\n\nTorbutton is now always enabled.\n\nClick on the Torbutton for more information.
+
+# Canvas permission prompt. Strings are kept here for ease of translation.
+canvas.siteprompt=This website (%S) attempted to access image data on a canvas. Since canvas image data can be used to discover information about your computer, blank image data was returned this time.
+canvas.allow=Allow in the Future
+canvas.allowAccessKey=A
+canvas.never=Never for This Site
+canvas.neverAccessKey=e