Mojo C++ bindings: support mapping T and T? differently.

services/ui/public/interfaces/window_tree.mojom

Index: services/ui/public/interfaces/window_tree.mojom
diff --git a/services/ui/public/interfaces/window_tree.mojom b/services/ui/public/interfaces/window_tree.mojom
index 65416a8f11a6e8c3366d75dd359020292e505ba9..e6548dc28208bee292e6a07e2c733582ddf8735b 100644
--- a/services/ui/public/interfaces/window_tree.mojom
+++ b/services/ui/public/interfaces/window_tree.mojom
@@ -102,9 +102,9 @@ interface WindowTree {
                 array<gfx.mojom.Rect>? additional_client_areas);
 
   // Mouse events outside a hit test mask do not hit the window. The |mask| is
-  // in window local coordinates. Pass null to clear the mask.
+  // in window local coordinates. Pass an empty rect to clear the mask.
   // TODO(jamescook): Convert |mask| to a path. http://crbug.com/613210
-  SetHitTestMask(uint32 window_id, gfx.mojom.Rect? mask);
+  SetHitTestMask(uint32 window_id, gfx.mojom.Rect mask);

[sky, 2016/07/26 21:26:16]

We want to allow null here too.

[yzshen1, 2016/07/26 21:32:36]

Please see my comment for the other place.

 
   // Sets the visibility of the specified window to |visible|. Connections are
   // allowed to change the visibility of any window they have created, as well