Introduce StickyKeys settings item with behind flag.

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index 81b7dbbf6896f23a5d647a859f00203f619e9a26..e9c67c56e55ad43a1193d1ddf06647ae5e8f402a 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -663,6 +663,9 @@ const char kEnableSavePasswordBubble[]      = "enable-save-password-bubble";
 // supported server-side for searches on google.com.
 const char kEnableSdch[]                    = "enable-sdch";
 
+// Enables support of sticky keys.
+const char kEnableStickyKeys[]              = "enable-sticky-keys";
+
 // Disable SPDY/3.1. This is a temporary testing flag.
 const char kDisableSpdy31[]                 = "disable-spdy31";