Change CaptureSynchronizer and PointerWatcherEventRouter to support multiple CaptureClients.

ui/aura/mus/capture_synchronizer.h

Index: ui/aura/mus/capture_synchronizer.h
diff --git a/ui/aura/mus/capture_synchronizer.h b/ui/aura/mus/capture_synchronizer.h
index 87bce872168d8c50d69ab3361b1d2643d8affb9b..4b300160f296e9d4718a8a587ef042eefaea1eb4 100644
--- a/ui/aura/mus/capture_synchronizer.h
+++ b/ui/aura/mus/capture_synchronizer.h
@@ -28,12 +28,11 @@ class CaptureClient;
 
 // CaptureSynchronizer is resonsible for keeping capture in sync between aura
 // and the mus server.
-class CaptureSynchronizer : public WindowObserver,
-                            public client::CaptureClientObserver {
+class AURA_EXPORT CaptureSynchronizer : public WindowObserver,
+                                        public client::CaptureClientObserver {
  public:
   CaptureSynchronizer(CaptureSynchronizerDelegate* delegate,
-                      ui::mojom::WindowTree* window_tree,
-                      client::CaptureClient* capture_client);
+                      ui::mojom::WindowTree* window_tree);
   ~CaptureSynchronizer() override;
 
   WindowMus* capture_window() { return capture_window_; }
@@ -41,6 +40,10 @@ class CaptureSynchronizer : public WindowObserver,
   // Called when the server side wants to change capture to |window|.
   void SetCaptureFromServer(WindowMus* window);
 
+  // Called when the |capture_client| has been set or will be unset.
+  void AttachToCaptureClient(client::CaptureClient* capture_client);
+  void DetachFromCaptureClient(client::CaptureClient* capture_client);
+
  private:
   // Internal implementation for capture changes. Adds/removes observer as
   // necessary and sets |capture_window_| to |window|.
@@ -54,7 +57,6 @@ class CaptureSynchronizer : public WindowObserver,
 
   CaptureSynchronizerDelegate* delegate_;
   ui::mojom::WindowTree* window_tree_;
-  client::CaptureClient* capture_client_;
 
   // Window that currently has capture.
   WindowMus* capture_window_ = nullptr;