Add an about:flag to support alternative data saver features

components/data_reduction_proxy/core/common/data_reduction_proxy_features.cc

Index: components/data_reduction_proxy/core/common/data_reduction_proxy_features.cc
diff --git a/components/data_reduction_proxy/core/common/data_reduction_proxy_features.cc b/components/data_reduction_proxy/core/common/data_reduction_proxy_features.cc
index fd4f3f60cca2f55e91c04d8d235687f79d8d032b..942004c1e3fa3a8161e5ed7d6043156d22061bb7 100644
--- a/components/data_reduction_proxy/core/common/data_reduction_proxy_features.cc
+++ b/components/data_reduction_proxy/core/common/data_reduction_proxy_features.cc
@@ -15,5 +15,9 @@ const base::Feature kDataReductionMainMenu{"DataReductionProxyMainMenu",
 const base::Feature kDataReductionSiteBreakdown{
     "DataReductionProxySiteBreakdown", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Enables an alternative Data Saver back end implementation.
+const base::Feature kDataReductionProxyAlternativeBackEnd{
+    "DataReductionProxyAlternativeBackEnd", base::FEATURE_DISABLED_BY_DEFAULT};
+
 }  // namespace features
 }  // namespace data_reduction_proxy