Added callback to DownloadUrl() so we can find download failures.

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 0f26b2c13a2e41d627faa06be4666a27475df179..ec1bd75a6c91e6c52fff510a5386c4c15a948b67 100644
--- a/content/browser/renderer_host/resource_dispatcher_host.cc
+++ b/content/browser/renderer_host/resource_dispatcher_host.cc
@@ -304,6 +304,19 @@ void OnSwapOutACKHelper(int render_process_id, int render_view_id) {
     rvh->OnSwapOutACK();
 }
 
+net::Error CallbackAndReturn(
+    const DownloadResourceHandler::OnStartedCallback& started_cb,
+    net::Error net_error) {
+  DCHECK(!started_cb.is_null());

[Randy Smith (Not in Mondays), 2012/03/09 19:14:02]

Why the DCHECK?  This may be a result of my not yet understanding callbacks
thoroughly, but I'm sure it's ok for people to pass in default constructed
callbacks (since you're doing it in this CL); if that translates to null, the
DCHECK shouldn't be here.  If it doesn't translate to null, the DCHECK is fine.

[ahendrickson, 2012/03/09 20:50:08]

Removed.

+  if (started_cb.is_null())
+    return net_error;
+  BrowserThread::PostTask(
+      BrowserThread::UI, FROM_HERE,
+      base::Bind(started_cb, content::DownloadId::Invalid(), net_error));
+
+  return net_error;
+}
+
 }  // namespace
 
 ResourceDispatcherHost* ResourceDispatcherHost::Get() {
@@ -924,7 +937,7 @@ net::Error ResourceDispatcherHost::BeginDownload(
     int route_id,
     content::ResourceContext* context) {
   if (is_shutdown_)
-    return net::ERR_INSUFFICIENT_RESOURCES;
+    return CallbackAndReturn(started_cb, net::ERR_INSUFFICIENT_RESOURCES);
 
   const GURL& url = request->original_url();
 #if defined(OS_CHROMEOS)
@@ -953,11 +966,13 @@ net::Error ResourceDispatcherHost::BeginDownload(
           CanRequestURL(child_id, url)) {
     VLOG(1) << "Denied unauthorized download request for "
             << url.possibly_invalid_spec();
-    return net::ERR_ACCESS_DENIED;
+    return CallbackAndReturn(started_cb, net::ERR_ACCESS_DENIED);
   }
 
   request_id_--;
 
+  // From this point forward, the |DownloadResourceHandler| is responsible for
+  // |started_cb|.
   scoped_refptr<ResourceHandler> handler(
       CreateResourceHandlerForDownload(request.get(), context, child_id,
                                        route_id, request_id_, save_info,