Initial implementation of Cronet Async API.

components/cronet/url_request_context_config.cc

Index: components/cronet/url_request_context_config.cc
diff --git a/components/cronet/url_request_context_config.cc b/components/cronet/url_request_context_config.cc
index e76bba3b1bb6168fc3b0c5df5811f0e3bb46151c..a644fbff486d1d6023f41e54ec25adc06e602d9b 100644
--- a/components/cronet/url_request_context_config.cc
+++ b/components/cronet/url_request_context_config.cc
@@ -55,7 +55,7 @@ void URLRequestContextConfig::ConfigureURLRequestContextBuilder(
   } else {
     context_builder->DisableHttpCache();
   }
-
+  context_builder->set_user_agent(user_agent);
   context_builder->SetSpdyAndQuicEnabled(enable_spdy, enable_quic);
   // TODO(mef): Use |config| to set cookies.
 }
@@ -73,6 +73,8 @@ void URLRequestContextConfig::RegisterJSONConverter(
                               &URLRequestContextConfig::http_cache_max_size);
   converter->RegisterStringField(REQUEST_CONTEXT_CONFIG_STORAGE_PATH,
                                  &URLRequestContextConfig::storage_path);
+  converter->RegisterStringField(REQUEST_CONTEXT_CONFIG_USER_AGENT,
+                                 &URLRequestContextConfig::user_agent);
   converter->RegisterRepeatedMessage(REQUEST_CONTEXT_CONFIG_QUIC_HINTS,
                                      &URLRequestContextConfig::quic_hints);
 }