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 d3cc1d8e0107f24004071a2109bd862af8564b1f..4d568f321c149cfee29c1a62a045cb734377b6c9 100644
--- a/services/ui/ws/frame_generator.h
+++ b/services/ui/ws/frame_generator.h
@@ -14,7 +14,7 @@
 #include "cc/surfaces/local_frame_id.h"
 #include "cc/surfaces/surface_sequence.h"
 #include "cc/surfaces/surface_sequence_generator.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"
 
@@ -46,7 +46,7 @@ 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,
                  scoped_refptr<DisplayCompositor> display_compositor);