Added probe to determine if data reduction proxy can be used

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 8de3ad959a02bbeecf0d5f321acb637d541d8a85..392a68270212734fd0d63781691a637efe352cbd 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -1300,6 +1300,9 @@ const char kSimulateOutdated[]               = "simulate-outdated";
 // Origin for which SpdyProxy authentication is supported.
 const char kSpdyProxyAuthOrigin[]           = "spdy-proxy-auth-origin";
 
+// Authentication string for the data reduction proxy.
+const char kSpdyProxyAuthValue[]            = "spdy-proxy-auth-value";
+
 // Speculative resource prefetching.
 const char kSpeculativeResourcePrefetching[] =
     "speculative-resource-prefetching";