CopyOutputRequest must have a working result_callback_ when received over mojo

cc/ipc/copy_output_request.mojom

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module cc.mojom;
 
+import "cc/ipc/copy_output_result.mojom";
 import "cc/ipc/texture_mailbox.mojom";
 import "mojo/common/unguessable_token.mojom";
 import "ui/gfx/geometry/mojo/geometry.mojom";
 
 // See cc/output/copy_output_request.h.
-// Note: result_callback_ is not included in this struct.
 struct CopyOutputRequest {
   mojo.common.mojom.UnguessableToken? source;
   bool force_bitmap_result;
   gfx.mojom.Rect? area;

[danakj, 2017/02/15 17:19:57]

btw Rect has no IsNull either

[Saman Sami, 2017/02/15 17:36:50]

We are using base::Optional<gfx::Rect> in CopyOutputRequest.

[danakj, 2017/02/15 17:41:59]

Hm, I mean to say that the ? here isn't doing anything though, right? If I
learnt how it works correctly last night. Cuz there's no null state for Rect
anyhow.

If the Rect is optionally present then it should be an Optional<Rect> here?

[Saman Sami, 2017/02/15 17:51:42]

Since base::Optional<gfx::Rect> is nullable, it does do something. Without ?, it
would crash if the rect does not exist.

[danakj, 2017/02/15 17:53:17]

I guess I still don't understand mojo nullable at all. :/

[Saman Sami, 2017/02/15 17:59:41]

If CopyOutputRequest had a gfx::Rect there would be no need for ?. But it has a
base::Optional<gfx::Rect> which is nullable and area() returns a
base::Optional<gfx::Rect> so we need to use ? otherwise when the base::Optional
does not have a gfx::Rect the message will be rejected.

-  cc.mojom.TextureMailbox? texture_mailbox;
+  TextureMailbox? texture_mailbox;
+  CopyOutputResultSender result_sender;
 };
+
+// When the display compositor is ready to respond to the CopyOutputRequest,
+// it uses this interface to send back the result.
+interface CopyOutputResultSender {
+  SendResult(CopyOutputResult result);
+};