Download: Check DownloadDriver guid before starting download.

Check whether core download system already has a guid through
DownloadDriver right before staring a download in the same function
scope.

Currently if core download system doesn't have guid, we first try to
get upload data in an asynchronous callback, then use a DCHECK to
ensure core download still doesn't have this guid. This is not
reliable.

Bug: 1303354
Change-Id: I5fd12ebf31541fe5c57c7265022954d0ecbbcfd1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3511085
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#979277}

components/download/internal/background_service/controller_impl.cc

@@ -1085,7 +1085,12 @@ void ControllerImpl::OnDownloadReadyToStart(
     return;
   }
 
-  DCHECK(!driver_->Find(guid).has_value());
+  auto driver_entry = driver_->Find(guid);
+  if (driver_entry.has_value()) {
+    DVLOG(1) << "Download already exists.";
+    return;
+  }
+
   driver_->Start(entry->request_params, entry->guid, entry->target_file_path,
                  post_body,
                  net::NetworkTrafficAnnotationTag(entry->traffic_annotation));