mus: Implement ScreenMus::GetCursorScreenPoint().

components/mus/public/interfaces/display.mojom

Index: components/mus/public/interfaces/display.mojom
diff --git a/components/mus/public/interfaces/display.mojom b/components/mus/public/interfaces/display.mojom
index 71364999b0ab75a82dc45714cd4507e0b101fc56..c22e45160e6fd834b3d1dda9081de26e836963a9 100644
--- a/components/mus/public/interfaces/display.mojom
+++ b/components/mus/public/interfaces/display.mojom
@@ -5,6 +5,7 @@
 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.
@@ -22,4 +23,8 @@ interface DisplayManagerObserver {
 
 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).

 };