mus: Add EventObserver to allow passively listening to UI events

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 eb2ef580ef31b7b8fb99e0eab0494d16f6976ed4..15eec8ca724d414da8083a18a97c0cad93773553 100644
--- a/components/mus/public/interfaces/window_tree.mojom
+++ b/components/mus/public/interfaces/window_tree.mojom
@@ -7,6 +7,7 @@ module mus.mojom;
 import "components/mus/public/interfaces/compositor_frame.mojom";
 import "components/mus/public/interfaces/cursor.mojom";
 import "components/mus/public/interfaces/input_events.mojom";
+import "components/mus/public/interfaces/input_event_matcher.mojom";
 import "components/mus/public/interfaces/mus_constants.mojom";
 import "components/mus/public/interfaces/surface_id.mojom";
 import "components/mus/public/interfaces/window_manager.mojom";
@@ -109,6 +110,13 @@ interface WindowTree {
   // if |window_id| does not currently have capture.
   ReleaseCapture(uint32 change_id, uint32 window_id);
 
+  // Sets an observer that monitors all events, even if they are not targeted
+  // at a window in this tree. If an event matchs |matcher| the observer reports
+  // it to the WindowTreeClient via OnEventObserved(). Each client may only have
+  // one observer at a time.
+  SetEventObserver(EventMatcher matcher);

[sky, 2016/04/21 17:23:03]

Declare this as EventMatcher?, then you can pass in null and don't need clear.

[James Cook, 2016/04/22 18:22:54]

Done.

+  ClearEventObserver();
+
   // Sets the specified bounds of the specified window.
   SetWindowBounds(uint32 change_id, uint32 window_id, mojo.Rect bounds);
 
@@ -362,6 +370,10 @@ interface WindowTreeClient {
   // the event is ack'ed.
   OnWindowInputEvent(uint32 event_id, uint32 window, Event event);
 
+  // Invoked when an event is sent via the EventObserver. Not targeted at a
+  // specific window. The client does not need to acknowledge these events.
+  OnEventObserved(Event event);
+
   // Called in two distinct cases: when a window known to the connection gains
   // focus, or when focus moves from a window known to the connection to a
   // window not known to the connection. In the later case |focused_window_id|