Delay generation of User-Agent header to URLRequestHttpJob and accept custom User-Agent from XHR/Fe…

content/common/resource_request.h

Index: content/common/resource_request.h
diff --git a/content/common/resource_request.h b/content/common/resource_request.h
index 83263cc5c1ccd40cdbae11011e65143a32e1fd3d..81b9316e370a5eb324cb149a655a5ad5455162fa 100644
--- a/content/common/resource_request.h
+++ b/content/common/resource_request.h
@@ -61,6 +61,10 @@ struct CONTENT_EXPORT ResourceRequest {
   // Additional HTTP request headers.
   std::string headers;
 
+  // The User-Agent string specified by the renderer to send in case |headers|
+  // results to get no entry named "User-Agent".
+  std::string default_user_agent;
+
   // net::URLRequest load flags (0 by default).
   int load_flags = 0;