Adds ability for WindowManager to create display roots

services/ui/ws/display.cc

Index: services/ui/ws/display.cc
diff --git a/services/ui/ws/display.cc b/services/ui/ws/display.cc
index 3513568e318a7d80f5525fa94f486e3f5d86a136..de4e8479d227e908826baf4de39e7c643de5a9c6 100644
--- a/services/ui/ws/display.cc
+++ b/services/ui/ws/display.cc
@@ -182,6 +182,18 @@ void Display::OnWillDestroyTree(WindowTree* tree) {
   }
 }
 
+void Display::RemoveWindowManagerDisplayRoot(
+    WindowManagerDisplayRoot* display_root) {
+  for (auto it = window_manager_display_root_map_.begin();
+       it != window_manager_display_root_map_.end(); ++it) {
+    if (it->second == display_root) {
+      window_manager_display_root_map_.erase(it);
+      return;
+    }
+  }
+  NOTREACHED();
+}
+
 void Display::UpdateNativeCursor(mojom::CursorType cursor_id) {
   if (cursor_id != last_cursor_) {
     platform_display_->SetCursorById(cursor_id);