mus: Implement ScreenMus::GetCursorScreenPoint().

components/mus/public/interfaces/display.mojom

 // Copyright 2016 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 mus.mojom;
 
 import "components/mus/public/interfaces/window_manager_constants.mojom";
+import "ui/mojo/geometry/geometry.mojom";
 
 interface DisplayManagerObserver {
   // Sent when the observer is added. Gives the initial state.
   OnDisplays(array<Display> displays);
 
   // The specified set of displays has changed in some way. This is used both
   // when a new display is added, or when a property of a display changes.
   // |displays| does not necessarily contain the complete set of displays,
   // only those that changed.
   OnDisplaysChanged(array<Display> displays);
 
   // An existing display was removed.
   OnDisplayRemoved(int64 id);
 };
 
 interface DisplayManager {
   AddObserver(DisplayManagerObserver observer);
+
+  // The mouse cursor on this display moved.
+  [Sync]
+  GetCursorScreenPoint() => (mojo.Point location);

[sky, 2016/04/15 20:38:57]

If you keep this on DisplayManager then there is events and cursor screen points
are on separate pipes and can be out of order. I recommend putting this on
WindowTreeHost to address that. Putting it on WindowTreeHost will simplify the
implementation too.

[Elliot Glaysher, 2016/04/15 23:56:52]

Done.

The call is now on WindowTree. I left the storage of the point in the
UserDisplayManager after poking at other places for it (notifying all the
WindowTreeHosts, putting it in the WindowServer, etc).

 };