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 c8d8ac3a9c50f8ce4c599ae1829dbe7aebb20a65..d29080e24a9261828abbca5770ce5300ed2bdf01 100644
--- a/content/common/url_loader.mojom
+++ b/content/common/url_loader.mojom
@@ -24,9 +24,15 @@ 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.

[dcheng, 2016/11/17 11:16:02]

Empty interfaces are a bit unusual. My main feedback is it would be useful for
the interface to explicitly describe why it's empty (it's an opaque handle),
which way the handle is passed (browser -> child), and explicitly what happens
when we close the pipe (the temporary file is deleted... I think?). Strawman
suggestion:

// Opaque handle passed from the browser process to a child
// process to manage the lifetime of temporary files used for
// download_to_file resource loading. When the message pipe
// for this interface is closed, the browser process will
// clean up the corresponding temporary file.

[tzik, 2016/11/22 13:54:45]

Done.

+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

[dcheng, 2016/11/17 11:16:02]

Nit: on => in

[tzik, 2016/11/22 13:54:45]

Done.

+  // the 'download_to_file' case.
+  OnReceiveResponse(URLResponseHead head, DownloadedTempFile? downloaded_file);
 
   // Called when the request has been redirected. The receiver is expected to
   // call FollowRedirect or cancel the request.