Add paper size reporting for CUPS printers (to let user to select one in the Print Preview).

printing/backend/print_backend.h

Index: printing/backend/print_backend.h
diff --git a/printing/backend/print_backend.h b/printing/backend/print_backend.h
index 3b307f1d9180e1dee99808a9123fc4ef07edee9f..e03ef42e37cf0785ba735f4a241310f70125a6e7 100644
--- a/printing/backend/print_backend.h
+++ b/printing/backend/print_backend.h
@@ -52,7 +52,8 @@ struct PRINTING_EXPORT PrinterSemanticCapsAndDefaults {
   ColorModel bw_model;
 
   struct Paper {
-    std::string name;
+    std::string display_name;
+    std::string vendor_id;
     gfx::Size size_um;
   };
   std::vector<Paper> papers;