Change CaptureSynchronizer and PointerWatcherEventRouter to support multiple CaptureClients.

ash/mus/window_manager.cc

Index: ash/mus/window_manager.cc
diff --git a/ash/mus/window_manager.cc b/ash/mus/window_manager.cc
index 8ef14a1046f117d1c972bf0704c521d36de9cd84..740e92f5b4166fa064f7028d2cd1a46e7cb47c89 100644
--- a/ash/mus/window_manager.cc
+++ b/ash/mus/window_manager.cc
@@ -114,6 +114,12 @@ void WindowManager::Init(
       std::move(frame_decoration_values));
 
   lookup_.reset(new WmLookupMus);
+
+  // Notify PointerWatcherEventRouter and CaptureSynchronizer that the capture
+  // client has been set.
+  aura::client::CaptureClient* capture_client = wm_state_->capture_controller();
+  capture_client->AddObserver(pointer_watcher_event_router_.get());
+  window_tree_client_->OnCaptureClientSet(capture_client);
 }
 
 void WindowManager::DeleteAllRootWindowControllers() {
@@ -238,6 +244,10 @@ void WindowManager::Shutdown() {
   if (!window_tree_client_)
     return;
 
+  aura::client::CaptureClient* capture_client = wm_state_->capture_controller();
+  capture_client->RemoveObserver(pointer_watcher_event_router_.get());
+  window_tree_client_->OnCaptureClientUnset(capture_client);
+
   Shell::DeleteInstance();
 
   lookup_.reset();