Add an about:flag to support alternative data saver features

components/data_reduction_proxy/core/common/data_reduction_proxy_headers.h

Index: components/data_reduction_proxy/core/common/data_reduction_proxy_headers.h
diff --git a/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.h b/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.h
index 360914e06ad1d029edd651d4d3c2ff997dcaecc3..1bae707c4e51452aa6733efcc8d71dad5c887313 100644
--- a/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.h
+++ b/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.h
@@ -98,9 +98,13 @@ const char* identity_directive();
 // preview requests and responses.
 const char* chrome_proxy_lite_page_directive();
 
-// Gets the Chrome-Proxy directive used by data reduction proxy lite page
-// preview experiment to ignore the blacklist.
-const char* chrome_proxy_lite_page_ignore_blacklist_directive();
+// Gets the Chrome-Proxy "exp" directive value used by data reduction proxy lite
+// page preview experiment to ignore the blacklist.
+const char* chrome_proxy_lite_page_ignore_blacklist();
+
+// Gets the Chrome-Proxy "exp" directive value used by data reduction proxy to
+// receive an alternative back end implementation.
+const char* chrome_proxy_alternative_back_end();
 
 // Requests a transformation only if the server determines that the page is
 // otherwise heavy (i.e., the associated page load ordinarily requires the