Add DataReductionProxyExperimentsStats and UMA for measuring potentially non-compressed bytes.

components/data_reduction_proxy/core/common/data_reduction_proxy_pref_names.cc

Index: components/data_reduction_proxy/core/common/data_reduction_proxy_pref_names.cc
diff --git a/components/data_reduction_proxy/core/common/data_reduction_proxy_pref_names.cc b/components/data_reduction_proxy/core/common/data_reduction_proxy_pref_names.cc
index a086117038ec4783bee2bb4d5de35752190007a9..0c8c657736c8a60021edd318017c24864d1dfd74 100644
--- a/components/data_reduction_proxy/core/common/data_reduction_proxy_pref_names.cc
+++ b/components/data_reduction_proxy/core/common/data_reduction_proxy_pref_names.cc
@@ -88,6 +88,13 @@ const char kHttpReceivedContentLength[] = "http_received_content_length";
 // received over the network.
 const char kHttpOriginalContentLength[] = "http_original_content_length";
 
+// Pref to store the expiration time of the last simulated Data Reduction Proxy
+// configuration. This is part of an experiment to see how many bytes are lost
+// if the Data Reduction Proxy is not used due to configuration being expired
+// or not available.
+const char kSimulatedConfigExpireTime[] =
+    "data_reduction.simulated_config_expire_time";
+
 // A boolean specifying whether the data reduction proxy statistics preferences
 // have migrated from local state to the profile.
 const char kStatisticsPrefsMigrated[] =