Initial implementation of Cronet Async API.

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 fef890177dcafa0446f6ce4f8e451f7e2f0f1a14..298939e3723ccba7ef8398b580533fe5f258bb35 100644
--- a/components/cronet/url_request_context_config.h
+++ b/components/cronet/url_request_context_config.h
@@ -43,6 +43,9 @@ struct URLRequestContextConfig {
   URLRequestContextConfig();
   ~URLRequestContextConfig();
 
+  // Load config values from JSON format.
+  bool LoadFromJSON(const std::string& config_string);
+
   // Configure |context_builder| based on |this|.
   void ConfigureURLRequestContextBuilder(
       net::URLRequestContextBuilder* context_builder);
@@ -62,6 +65,8 @@ struct URLRequestContextConfig {
   int http_cache_max_size;
   // Storage path for http cache and cookie storage.
   std::string storage_path;
+  // User-Agent request header field.
+  std::string user_agent;
   // App-provided list of servers that support QUIC.
   ScopedVector<QuicHint> quic_hints;