Candidate fix for observe-window-after-free error in WindowTreeClientImpl::LocalSetCapture

components/mus/public/cpp/lib/window_tree_client_impl.h

Index: components/mus/public/cpp/lib/window_tree_client_impl.h
diff --git a/components/mus/public/cpp/lib/window_tree_client_impl.h b/components/mus/public/cpp/lib/window_tree_client_impl.h
index aef326f7cc320d143bfa1368f37c5eb1f81415f7..6c20caedb544fd6db89d82b85ec50b351fe77c45 100644
--- a/components/mus/public/cpp/lib/window_tree_client_impl.h
+++ b/components/mus/public/cpp/lib/window_tree_client_impl.h
@@ -133,6 +133,7 @@ class WindowTreeClientImpl : public WindowTreeConnection,
 
  private:
   friend class WindowTreeClientImplPrivate;
+  class MusWindowObserver;
 
   enum class NewWindowType {
     CHILD,
@@ -147,6 +148,8 @@ class WindowTreeClientImpl : public WindowTreeConnection,
   // Returns the oldest InFlightChange that matches |change|.
   InFlightChange* GetOldestInFlightChangeMatching(const InFlightChange& change);
 
+  void SetCaptureWindow(Window* window);
+
   // See InFlightChange for details on how InFlightChanges are used.
   uint32_t ScheduleInFlightChange(std::unique_ptr<InFlightChange> change);
 
@@ -288,7 +291,7 @@ class WindowTreeClientImpl : public WindowTreeConnection,
 
   IdToWindowMap windows_;
 
-  Window* capture_window_;
+  std::unique_ptr<MusWindowObserver> capture_window_observer_;
 
   Window* focused_window_;