Add an about:flag to support alternative data saver features

components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc

Index: components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc
diff --git a/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc b/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc
index d8a021928967a12591b5460e750a99d0d08a8048..783430f7c60876cfcef93bf88b7c407be60725d4 100644
--- a/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc
+++ b/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc
@@ -46,8 +46,8 @@ const char kIdentityDirective[] = "identity";
 // The legacy Chrome-Proxy response header directive for LoFi images.
 const char kLegacyChromeProxyLoFiResponseDirective[] = "q=low";
 
-const char kChromeProxyLitePageIngoreBlacklistDirective[] =
-    "exp=ignore_preview_blacklist";
+const char kChromeProxyLitePageIngoreBlacklist[] = "ignore_preview_blacklist";
+const char kChromeProxyAlternativeBackEnd[] = "alternative";
 
 const char kIfHeavyQualifier[] = "if-heavy";
 
@@ -155,8 +155,12 @@ const char* identity_directive() {
   return kIdentityDirective;
 }
 
-const char* chrome_proxy_lite_page_ignore_blacklist_directive() {
-  return kChromeProxyLitePageIngoreBlacklistDirective;
+const char* chrome_proxy_lite_page_ignore_blacklist() {
+  return kChromeProxyLitePageIngoreBlacklist;
+}
+
+const char* chrome_proxy_alternative_back_end() {
+  return kChromeProxyAlternativeBackEnd;
 }
 
 const char* if_heavy_qualifier() {