Add histograms for requests that were fetched successfully

components/data_reduction_proxy/core/common/data_reduction_proxy_params.h

Index: components/data_reduction_proxy/core/common/data_reduction_proxy_params.h
diff --git a/components/data_reduction_proxy/core/common/data_reduction_proxy_params.h b/components/data_reduction_proxy/core/common/data_reduction_proxy_params.h
index 61f9bb105e2a2447cbee37e5cd619c5eb4f239e0..1cdc84ebd90d3bd65f6689825c2c083187a74342 100644
--- a/components/data_reduction_proxy/core/common/data_reduction_proxy_params.h
+++ b/components/data_reduction_proxy/core/common/data_reduction_proxy_params.h
@@ -144,14 +144,14 @@ const char* GetServerExperimentsFieldTrialName();
 }  // namespace params
 
 // Contains information about a given proxy server. |proxies_for_http| contains
-// the configured data reduction proxy servers. |is_fallback| notes whether the
-// given proxy is a fallback.
+// the configured data reduction proxy servers. |proxy_index| notes the index
+// of the data reduction proxy used in the list of all data reduction proxies.
 struct DataReductionProxyTypeInfo {
   DataReductionProxyTypeInfo();
   DataReductionProxyTypeInfo(const DataReductionProxyTypeInfo& other);
   ~DataReductionProxyTypeInfo();
   std::vector<net::ProxyServer> proxy_servers;
-  bool is_fallback;
+  size_t proxy_index;
 };
 
 // Provides initialization parameters. Proxy origins, and the secure proxy