Add DataReductionProxy IPC to determine if the Data Reduction Proxy was used.

content/public/renderer/document_state.h

Index: content/public/renderer/document_state.h
diff --git a/content/public/renderer/document_state.h b/content/public/renderer/document_state.h
index d388a8a979ef0dcdc695ac86221ce470409ffce7..bd97cdaf55727d8c9b9cfa99665419f368edf455 100644
--- a/content/public/renderer/document_state.h
+++ b/content/public/renderer/document_state.h
@@ -161,7 +161,7 @@ class CONTENT_EXPORT DocumentState
     was_fetched_via_proxy_ = value;
   }
 
-  net::HostPortPair proxy_server() const { return proxy_server_; }
+  net::HostPortPair& proxy_server() { return proxy_server_; }
   void set_proxy_server(net::HostPortPair proxy_server) {
     proxy_server_ = proxy_server;
   }