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 e4855ba7dcabf16b3736454f522c26aea38c9fc0..e5a15f16c23aee0782e43a591ccf288a242eacd7 100644
--- a/components/mus/public/interfaces/window_tree.mojom
+++ b/components/mus/public/interfaces/window_tree.mojom
@@ -170,11 +170,12 @@ 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| on top of |window_id|.
   // 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|.
+  // . |transient_window_id| is modal to system.
   AddTransientWindow(uint32 change_id,
                      uint32 window_id,
                      uint32 transient_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. 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.
+  // 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