Gather UMA for click retarget due to DOM changes

third_party/WebKit/Source/core/input/MouseEventManager.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 MouseEventManager_h
 #define MouseEventManager_h
 
 #include "core/CoreExport.h"
 #include "core/dom/SynchronousMutationObserver.h"
 #include "core/input/BoundaryEventDispatcher.h"
@@ skipping 208 matching lines @@
 
   unsigned mouse_down_may_start_autoscroll_ : 1;
   unsigned svg_pan_ : 1;
   unsigned captures_dragging_ : 1;
   unsigned mouse_down_may_start_drag_ : 1;
 
   Member<Node> mouse_press_node_;
 
   int click_count_;
   Member<Element> click_element_;
+  // This element should be mostly the same as click_element_. Only when
+  // click_element_ is set to null due to DOM manipulation mouse_down_element_
+  // remains unchanged.
+  Member<Element> mouse_down_element_;
 
   IntPoint mouse_down_pos_;  // In our view's coords.
   TimeTicks mouse_down_timestamp_;
   WebMouseEvent mouse_down_;
 
   LayoutPoint drag_start_pos_;
 
   TaskRunnerTimer<MouseEventManager> fake_mouse_move_event_timer_;
 };
 
 }  // namespace blink
 
 #endif  // MouseEventManager_h