mus: Fix a teardown crash.

services/ui/ws/frame_generator.h

Index: services/ui/ws/frame_generator.h
diff --git a/services/ui/ws/frame_generator.h b/services/ui/ws/frame_generator.h
index 3f2f8af5006776b223399588dd12479da93f334d..bf80ba544ef067698af90d0b41d9c3a5f05e9a8f 100644
--- a/services/ui/ws/frame_generator.h
+++ b/services/ui/ws/frame_generator.h
@@ -15,7 +15,7 @@
 #include "cc/surfaces/surface_sequence.h"
 #include "cc/surfaces/surface_sequence_generator.h"
 #include "services/ui/ws/ids.h"
-#include "services/ui/ws/server_window_observer.h"
+#include "services/ui/ws/server_window_tracker.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/native_widget_types.h"
 
@@ -47,7 +47,7 @@ class ServerWindowCompositorFrameSink;
 
 // Responsible for redrawing the display in response to the redraw requests by
 // submitting CompositorFrames to the owned CompositorFrameSink.
-class FrameGenerator : public ServerWindowObserver {
+class FrameGenerator : public ServerWindowTracker {
  public:
   FrameGenerator(FrameGeneratorDelegate* delegate,
                  ServerWindow* root_window,