Add Net.ProxyScriptFetchSuccessDuration UMA to record how long PAC script fetches take.

net/proxy/proxy_script_fetcher_impl.h

Index: net/proxy/proxy_script_fetcher_impl.h
diff --git a/net/proxy/proxy_script_fetcher_impl.h b/net/proxy/proxy_script_fetcher_impl.h
index 2afb11651a2fa1181e2fca551779a3073fbb544d..85ad7a08a139a31e6e6619b13538a1152229db39 100644
--- a/net/proxy/proxy_script_fetcher_impl.h
+++ b/net/proxy/proxy_script_fetcher_impl.h
@@ -116,6 +116,9 @@ class NET_EXPORT ProxyScriptFetcherImpl : public ProxyScriptFetcher,
   // The maximum amount of time to wait for download to complete.
   base::TimeDelta max_duration_;
 
+  // The time that the fetch started.
+  base::Time fetch_start_time_;
+
   // Factory for creating the time-out task. This takes care of revoking
   // outstanding tasks when |this| is deleted.
   base::WeakPtrFactory<ProxyScriptFetcherImpl> weak_factory_;