Wires up drag/drop for aura-mus

ui/base/window_tracker_template.h

Index: ui/base/window_tracker_template.h
diff --git a/ui/base/window_tracker_template.h b/ui/base/window_tracker_template.h
index 5bcaccb40bd4f325cd5814bfaa69fc0b2fc11c73..e91bdac8077b29cd53cc9aca13dbf753b25e3201 100644
--- a/ui/base/window_tracker_template.h
+++ b/ui/base/window_tracker_template.h
@@ -30,10 +30,7 @@ class WindowTrackerTemplate : public TObserver {
       Add(window);
   }
   WindowTrackerTemplate() {}
-  ~WindowTrackerTemplate() override {
-    for (T* window : windows_)
-      window->RemoveObserver(this);
-  }
+  ~WindowTrackerTemplate() override { RemoveAll(); }
 
   // Returns the set of windows being observed.
   const WindowList& windows() const { return windows_; }
@@ -47,6 +44,12 @@ class WindowTrackerTemplate : public TObserver {
     windows_.push_back(window);
   }
 
+  void RemoveAll() {
+    for (T* window : windows_)
+      window->RemoveObserver(this);
+    windows_.clear();
+  }
+
   // Removes |window| from the set of windows being tracked.
   void Remove(T* window) {
     auto iter = std::find(windows_.begin(), windows_.end(), window);