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..d3d8c54064a449998b9715bcf0833510bfc9ebb1 100644
--- a/content/public/renderer/document_state.h
+++ b/content/public/renderer/document_state.h
@@ -161,8 +161,8 @@ class CONTENT_EXPORT DocumentState
     was_fetched_via_proxy_ = value;
   }
 
-  net::HostPortPair proxy_server() const { return proxy_server_; }
-  void set_proxy_server(net::HostPortPair proxy_server) {
+  const net::HostPortPair& proxy_server() const { return proxy_server_; }
+  void set_proxy_server(const net::HostPortPair& proxy_server) {
     proxy_server_ = proxy_server;
   }