Remove PlatformEvent it is no longer used.

third_party/WebKit/Source/core/events/MouseEvent.h

Index: third_party/WebKit/Source/core/events/MouseEvent.h
diff --git a/third_party/WebKit/Source/core/events/MouseEvent.h b/third_party/WebKit/Source/core/events/MouseEvent.h
index 831be6c723f3fa9bcb95a19194255f880e305b90..a86671562c266194dc846674fadc76f0df5a7240 100644
--- a/third_party/WebKit/Source/core/events/MouseEvent.h
+++ b/third_party/WebKit/Source/core/events/MouseEvent.h
@@ -69,7 +69,7 @@ class CORE_EXPORT MouseEvent : public UIEventWithKeyState {
 
   ~MouseEvent() override;
 
-  static unsigned short platformModifiersToButtons(unsigned modifiers);
+  static unsigned short webInputEventModifiersToButtons(unsigned modifiers);
 
   void initMouseEvent(ScriptState*,
                       const AtomicString& type,
@@ -202,7 +202,7 @@ class CORE_EXPORT MouseEvent : public UIEventWithKeyState {
              int windowY,
              int movementX,
              int movementY,
-             PlatformEvent::Modifiers,
+             WebInputEvent::Modifiers,
              short button,
              unsigned short buttons,
              EventTarget* relatedTarget,
@@ -227,7 +227,7 @@ class CORE_EXPORT MouseEvent : public UIEventWithKeyState {
                               int screenY,
                               int clientX,
                               int clientY,
-                              PlatformEvent::Modifiers,
+                              WebInputEvent::Modifiers,
                               short button,
                               EventTarget* relatedTarget,
                               InputDeviceCapabilities* sourceCapabilities,