mus: Use the new drag API to implement tab dragging in chrome.

chrome/browser/ui/views/tabs/window_finder.h

 // Copyright 2014 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 CHROME_BROWSER_UI_VIEWS_TABS_WINDOW_FINDER_H_
 #define CHROME_BROWSER_UI_VIEWS_TABS_WINDOW_FINDER_H_
 
 #include <set>
 
 #include "ui/gfx/native_widget_types.h"
 
 namespace aura {
 class Window;
 }
 
 namespace gfx {
 class Point;
 }
 
+// Class used by the tabstrip to find chrome windows that we can attach tabs
+// to.
 class WindowFinder {
  public:
   WindowFinder();
   virtual ~WindowFinder();
 
+  // Finds the topmost chrome window at |screen_point|. Ignores the windows in
+  // |ignore|, which contain windows such as the tab being dragged right now.

[sky, 2016/07/01 16:01:16]

Can you also add the point you weren't aware of? Perhaps something like:
If a window from another app is the topmost window at |screen_point|, null is
returned.

   virtual gfx::NativeWindow GetLocalProcessWindowAtPoint(
       const gfx::Point& screen_point,
       const std::set<gfx::NativeWindow>& ignore);
 
  private:
   DISALLOW_COPY_AND_ASSIGN(WindowFinder);
 };
 
 #endif  // CHROME_BROWSER_UI_VIEWS_TABS_WINDOW_FINDER_H_