Add support for X11 workspaces

ui/aura/window_tree_host_observer.h

Index: ui/aura/window_tree_host_observer.h
diff --git a/ui/aura/window_tree_host_observer.h b/ui/aura/window_tree_host_observer.h
index d96e9d3b24dc1230deb2ab5920787bcbff2db98f..257bd553504927686f33a1cdf9c8428b00e37fb7 100644
--- a/ui/aura/window_tree_host_observer.h
+++ b/ui/aura/window_tree_host_observer.h
@@ -24,6 +24,10 @@ class AURA_EXPORT WindowTreeHostObserver {
   virtual void OnHostMoved(const WindowTreeHost* host,
                            const gfx::Point& new_origin) {}
 
+  // Called when the host is moved on screen.
+  virtual void OnHostWorkspaceChanged(const WindowTreeHost* host,
+                                      int workspace_id) {}
+
   // Called when the native window system sends the host request to close.
   virtual void OnHostCloseRequested(const WindowTreeHost* host) {}