Populates sourceDevice attribute into MouseEvent

Source/core/events/MouseRelatedEvent.h

 /*
  * Copyright (C) 2001 Peter Kelly (pmk@post.com)
  * Copyright (C) 2001 Tobias Anton (anton@stud.fbi.fh-darmstadt.de)
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006 Apple Computer, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 47 matching lines @@
 
         // Page point in "absolute" coordinates (i.e. post-zoomed, page-relative coords,
         // usable with LayoutObject::absoluteToLocal).
         const LayoutPoint& absoluteLocation() const { return m_absoluteLocation; }
         void setAbsoluteLocation(const LayoutPoint& p) { m_absoluteLocation = p; }
 
         DECLARE_VIRTUAL_TRACE();
 
     protected:
         MouseRelatedEvent();
+        // TODO(lanwei): Will make this argument non-optional and all the callers need to provide
+        // sourceDevice even when it is null, see https://crbug.com/476530.
         MouseRelatedEvent(const AtomicString& type, bool canBubble, bool cancelable,
             PassRefPtrWillBeRawPtr<AbstractView>, int detail, const IntPoint& screenLocation,
             const IntPoint& rootFrameLocation, const IntPoint& movementDelta, bool ctrlKey, bool altKey,
-            bool shiftKey, bool metaKey, bool isSimulated = false);
+            bool shiftKey, bool metaKey, bool isSimulated = false, InputDevice* sourceDevice = nullptr);
 
         void initCoordinates();
         void initCoordinates(const LayoutPoint& clientLocation);
         virtual void receivedTarget() override final;
 
         void computePageLocation();
         void computeRelativePosition();
 
         // Expose these so MouseEvent::initMouseEvent can set them.
         IntPoint m_screenLocation;
         LayoutPoint m_clientLocation;
         LayoutPoint m_movementDelta;
 
     private:
         LayoutPoint m_pageLocation;
         LayoutPoint m_layerLocation;
         LayoutPoint m_offsetLocation;
         LayoutPoint m_absoluteLocation;
         bool m_isSimulated;
         bool m_hasCachedRelativePosition;
     };
 
 } // namespace blink
 
 #endif // MouseRelatedEvent_h