[CUPS] WebUI handler for CUPS printing: Retrieve the printer list from user preference.

url/url_constants.h

Index: url/url_constants.h
diff --git a/url/url_constants.h b/url/url_constants.h
index 3a423d2634201ba18aa1578db9f8500c2f3ff881..0fe4961700ce8563b14f5f5a58de6241dc2d1def 100644
--- a/url/url_constants.h
+++ b/url/url_constants.h
@@ -29,6 +29,12 @@ URL_EXPORT extern const char kJavaScriptScheme[];
 URL_EXPORT extern const char kMailToScheme[];
 URL_EXPORT extern const char kWsScheme[];
 URL_EXPORT extern const char kWssScheme[];
+// Below are the schemes for the network printers. For more info, see
+// https://www.cups.org/doc/network.html#PROTOCOLS.
+URL_EXPORT extern const char kPrinterIppScheme[];
+URL_EXPORT extern const char kPrinterIppsScheme[];
+URL_EXPORT extern const char kPrinterLpdScheme[];
+URL_EXPORT extern const char kPrinterSocketScheme[];
 
 // Used to separate a standard scheme and the hostname: "://".
 URL_EXPORT extern const char kStandardSchemeSeparator[];