Wires up drag/drop for aura-mus

ui/base/window_tracker_template.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_BASE_WINDOW_TRACKER_TEMPLATE_H_
 #define UI_BASE_WINDOW_TRACKER_TEMPLATE_H_
 
 #include <vector>
 
 #include "base/macros.h"
@@ skipping 12 matching lines @@
   // A vector<> is used for tracking the windows (instead of a set<>) because
   // the user may want to know about the order of the windows that have been
   // added.
   using WindowList = std::vector<T*>;
 
   explicit WindowTrackerTemplate(const WindowList& windows) {
     for (T* window : windows)
       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_; }
 
   // Adds |window| to the set of Windows being tracked.
   void Add(T* window) {
     if (base::ContainsValue(windows_, window))
       return;
 
     window->AddObserver(this);
     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);
     if (iter != windows_.end()) {
       window->RemoveObserver(this);
       windows_.erase(iter);
     }
   }
 
   T* Pop() {
@@ skipping 17 matching lines @@
 
  private:
   WindowList windows_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTrackerTemplate);
 };
 
 }  // namespace ui
 
 #endif  // UI_BASE_WINDOW_TRACKER_TEMPLATE_H_