mus Window Server: implement event capture

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 73858e2b6de6e0a5b6492d51477338e75bbb146b..b600ba47acb0a5751be22162943664d67c50bdd2 100644
--- a/components/mus/public/interfaces/window_tree.mojom
+++ b/components/mus/public/interfaces/window_tree.mojom
@@ -105,6 +105,17 @@ interface WindowTree {
   // can delete it.
   DeleteWindow(uint32 change_id, uint32 window_id);
 
+  // Requests input event capture for the given |window_id|. Capture is only
+  // allowed if the window is processing an event. When a window gains capture,
+  // current input events are canceled. The given window will receive all
+  // subsequent input until an alternate window is set via SetCapture, or
+  // ReleaseCapture is called for |window_id|. OnLostCapture is called to notify
+  // of capture ending.
+  SetCapture(uint32 change_id, uint32 window_id);
+
+  // Releases input event capture for the given |window_id|.

[sky, 2016/02/08 16:57:11]

Document this does nothing if |window_id| does not currently have capture.

[jonross, 2016/02/10 15:57:56]

Done.

+  ReleaseCapture(uint32 change_id, uint32 window_id);
+
   // Sets the specified bounds of the specified window.
   SetWindowBounds(uint32 change_id, uint32 window_id, mojo.Rect bounds);
 
@@ -265,6 +276,9 @@ interface WindowTreeClient {
   // previously embedded in. See Embed() for more information.
   OnUnembed(uint32 window);
 
+  // Sent when a window loses capture.
+  OnLostCapture(uint32 window);
+
   // Called in response to NewTopLevelWindow() successfully completing.
   OnTopLevelCreated(uint32 change_id, WindowData data);