Add pending preloads indexed by routing id. Start preloading once we navigate.

content/browser/renderer_host/resource_dispatcher_host.cc

Index: content/browser/renderer_host/resource_dispatcher_host.cc
diff --git a/content/browser/renderer_host/resource_dispatcher_host.cc b/content/browser/renderer_host/resource_dispatcher_host.cc
index aebdbb045e7a7e7503d95e542fcce1b5383f1501..e132eba7364d0b8f04321be8422b4673d227058a 100644
--- a/content/browser/renderer_host/resource_dispatcher_host.cc
+++ b/content/browser/renderer_host/resource_dispatcher_host.cc
@@ -466,10 +466,14 @@ void ResourceDispatcherHost::BeginRequest(
   // be prerendered. This should be in front of the [a]syncResourceHandler,
   // but after the BufferedResourceHandler since it depends on the MIME
   // sniffing capabilities in the BufferedResourceHandler.
+  bool is_from_prerender = ((load_flags & net::LOAD_PRERENDER) != 0);
   prerender::PrerenderResourceHandler* pre_handler =
       prerender::PrerenderResourceHandler::MaybeCreate(*request,
                                                        context,
-                                                       handler);
+                                                       handler,
+                                                       is_from_prerender,
+                                                       child_id,
+                                                       route_id);
   if (pre_handler)
     handler = pre_handler;