Fix and refactor downloaded file handling in the loading stack

content/common/url_loader.mojom

Index: content/common/url_loader.mojom
diff --git a/content/common/url_loader.mojom b/content/common/url_loader.mojom
index f66e93c97371e2a7a37f7e0399a841a4801cddb4..d238b16f9dedece326070e43f264b610460d23c0 100644
--- a/content/common/url_loader.mojom
+++ b/content/common/url_loader.mojom
@@ -21,9 +21,15 @@ interface URLLoader {
   FollowRedirect();
 };
 
+// Represents a temporary file for a download_to_file case of resource loading.
+// The downloaded file keeps alive until the instance is destroyed.
+interface DownloadedTempFile {
+};
+
 interface URLLoaderClient {
-  // Called when the response head is received.
-  OnReceiveResponse(URLResponseHead head);
+  // Called when the response head is received. |downloaded_file| is non-null on
+  // the 'download_to_file' case.
+  OnReceiveResponse(URLResponseHead head, DownloadedTempFile? downloaded_file);
 
   // Called when some data from a resource request has been downloaded to the
   // file. This is only called in the 'download_to_file' case and replaces