Reland: mus: Enable system modal windows

components/mus/public/interfaces/window_tree.mojom

Index: components/mus/public/interfaces/window_tree.mojom
diff --git a/components/mus/public/interfaces/window_tree.mojom b/components/mus/public/interfaces/window_tree.mojom
index f899c62f81e65cd9b66e6cb6943dc8e64b22f5eb..8df1a4ac85e12c4be591543ae7da2f342142e010 100644
--- a/components/mus/public/interfaces/window_tree.mojom
+++ b/components/mus/public/interfaces/window_tree.mojom
@@ -170,8 +170,9 @@ interface WindowTree {
   // visible to connection B.
   RemoveWindowFromParent(uint32 change_id, uint32 window_id);
 
-  // Ties the lifetime of |child| to the lifetime of |parent|. This also
-  // places |child| always on top of |parent|.
+  // Ties the lifetime of |transient_window_id| to the lifetime of |window_id|.
+  // This also places |transient_window_id| always on top of |window_id|.

[sky, 2016/04/29 00:23:18]

nit: remove 'always'.

[mohsen, 2016/04/29 03:33:37]

Done.

+  // |transient_window_id| should not be a system modal window.

[sky, 2016/04/29 00:23:18]

Fails if |transient|window_id| is a system modal window.

[mohsen, 2016/04/29 03:33:37]

I think "Fails if" is used in comments whenever the action fails gracefully such
that the client is notified of the failure to revert the client-side value.
That's not the case here as we have a DCHECK. Right?

   // This fails for any of the following reasons:
   // . |window_id| or |transient_window_id| does not identify a valid window.
   // . |transient_window_id| is an ancestor of |window_id|.
@@ -183,11 +184,12 @@ interface WindowTree {
   // This does not change transient window's position in the window hierarchy.
   RemoveTransientWindowFromParent(uint32 change_id, uint32 transient_window_id);
 
-  // Sets |window_id| to be modal to its transient parent.
-  // This fails if |window_id| does not identify a valid window.
-  // TODO(mohsen): If |window_id| does not have a transient parent, this will
-  // have no effect. Plan is to make a window modal to system if it does not
-  // have a transient parent.
+  // Sets |window_id| to be modal. It would be modal to its transient parent if

[sky, 2016/04/29 00:23:18]

If the window has a transient parent, then the window is modal to the transient
parent. Otherwise the window is modal to the system. This releases capture if
necessary.

[mohsen, 2016/04/29 03:33:37]

Done.

+  // it has one. Otherwise, it would be modal to the whole system. It will also
+  // release capture if necessary.
+  // This fails for any of the following reasons:
+  // . |window_id| does not identify a valid window.
+  // . Client does not have a valid user id (i.e., it is an embedded app).
   SetModal(uint32 change_id, uint32 window_id);
 
   // Reorders a window in its parent, relative to |relative_window_id| according