Add Cronet experimental option for host cache persistence

components/cronet/url_request_context_config.h

Index: components/cronet/url_request_context_config.h
diff --git a/components/cronet/url_request_context_config.h b/components/cronet/url_request_context_config.h
index 7b13833b528f1485c3c576facd558a14d82b3c31..2a2aef0c27d183bd4716a6b7e5e17f7ecfc6fd62 100644
--- a/components/cronet/url_request_context_config.h
+++ b/components/cronet/url_request_context_config.h
@@ -167,6 +167,13 @@ struct URLRequestContextConfig {
   // The list of public key pins.
   std::vector<std::unique_ptr<Pkp>> pkp_list;
 
+  // Enable DNS cache persistence.
+  bool enable_host_cache_persistence = false;
+
+  // Minimum time in milliseconds between writing the HostCache contents to
+  // prefs.

[pauljensen, 2017/06/26 16:37:18]

nit: might want to mention this value only has significance when
|enable_host_cache_persistence| is true.

[mgersh, 2017/06/26 23:21:45]

Done.

+  int host_cache_persistence_timer = 60000;

[pauljensen, 2017/06/26 16:37:17]

"timer" seems like an active name for a variable that holds a value, WDYT of
less active sounding name, like "interval"?

[pauljensen, 2017/06/26 16:37:17]

could we add a unit suffix, like "_ms"?

[mgersh, 2017/06/26 23:21:45]

Done. (I used "delay" instead of "interval" because that's what I called it in
HostCachePersistenceManager.)

[mgersh, 2017/06/26 23:21:45]

Done.

+
   // Experimental options that are recognized by the config parser.
   std::unique_ptr<base::DictionaryValue> effective_experimental_options =
       nullptr;