Remove PlatformEvent it is no longer used.

third_party/WebKit/Source/core/events/MouseEvent.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, 2008 Apple Inc. All rights reserved.
  *
  * 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 51 matching lines @@
                             const AtomicString& eventType,
                             const MouseEventInit&);
 
   static MouseEvent* create(const AtomicString& eventType,
                             AbstractView*,
                             Event* underlyingEvent,
                             SimulatedClickCreationScope);
 
   ~MouseEvent() override;
 
-  static unsigned short platformModifiersToButtons(unsigned modifiers);
+  static unsigned short webInputEventModifiersToButtons(unsigned modifiers);
 
   void initMouseEvent(ScriptState*,
                       const AtomicString& type,
                       bool canBubble,
                       bool cancelable,
                       AbstractView*,
                       int detail,
                       int screenX,
                       int screenY,
                       int clientX,
@@ skipping 112 matching lines @@
              bool canBubble,
              bool cancelable,
              AbstractView*,
              int detail,
              int screenX,
              int screenY,
              int windowX,
              int windowY,
              int movementX,
              int movementY,
-             PlatformEvent::Modifiers,
+             WebInputEvent::Modifiers,
              short button,
              unsigned short buttons,
              EventTarget* relatedTarget,
              TimeTicks platformTimeStamp,
              SyntheticEventType,
              const String& region);
 
   MouseEvent(const AtomicString& type, const MouseEventInit&);
 
   MouseEvent();
 
   short rawButton() const { return m_button; }
 
  private:
   friend class MouseEventDispatchMediator;
   void initMouseEventInternal(const AtomicString& type,
                               bool canBubble,
                               bool cancelable,
                               AbstractView*,
                               int detail,
                               int screenX,
                               int screenY,
                               int clientX,
                               int clientY,
-                              PlatformEvent::Modifiers,
+                              WebInputEvent::Modifiers,
                               short button,
                               EventTarget* relatedTarget,
                               InputDeviceCapabilities* sourceCapabilities,
                               unsigned short buttons = 0);
 
   void initCoordinates(const double clientX, const double clientY);
   void initCoordinatesFromRootFrame(int windowX, int windowY);
   void receivedTarget() final;
 
   void computePageLocation();
@@ skipping 26 matching lines @@
   MouseEvent& event() const;
 
   DispatchEventResult dispatchEvent(EventDispatcher&) const override;
 };
 
 DEFINE_EVENT_TYPE_CASTS(MouseEvent);
 
 }  // namespace blink
 
 #endif  // MouseEvent_h