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

url/url_constants.cc

Index: url/url_constants.cc
diff --git a/url/url_constants.cc b/url/url_constants.cc
index 549819e97f3abdbe6ee24aac569c9b6387f2520e..02854f2258fce1b3f3f620206e6a7cea15c22656 100644
--- a/url/url_constants.cc
+++ b/url/url_constants.cc
@@ -23,6 +23,10 @@ const char kJavaScriptScheme[] = "javascript";
 const char kMailToScheme[] = "mailto";
 const char kWsScheme[] = "ws";
 const char kWssScheme[] = "wss";
+const char kPrinterIppScheme[] = "ipp";
+const char kPrinterIppsScheme[] = "ipps";
+const char kPrinterLpdScheme[] = "lpd";
+const char kPrinterSocketScheme[] = "socket";
 
 const char kStandardSchemeSeparator[] = "://";