Remove FrameSinkId hierarchy when unparenting ServerWindow.

services/ui/ws/server_window.cc

Index: services/ui/ws/server_window.cc
diff --git a/services/ui/ws/server_window.cc b/services/ui/ws/server_window.cc
index 64f79e26433a72fb031b6da5422cd2adbf15dcc3..654b40aab2a22aedda40373387ca9519a25e850b 100644
--- a/services/ui/ws/server_window.cc
+++ b/services/ui/ws/server_window.cc
@@ -146,8 +146,12 @@ void ServerWindow::Remove(ServerWindow* child) {
 
   for (auto& observer : child->observers_)
     observer.OnWillChangeWindowHierarchy(child, nullptr, this);
+
   RemoveImpl(child);
 
+  if (GetRoot() != nullptr)
+    child->ProcessRootChanged(GetRoot(), nullptr);
+
   // Stack the child properly if it is a transient child of a sibling.
   if (child->transient_parent_ && child->transient_parent_->parent() == this)
     RestackTransientDescendants(child->transient_parent_, &GetStackingTarget,