Remove PlatformEvent it is no longer used.

third_party/WebKit/Source/core/input/MouseEventManager.cpp

Index: third_party/WebKit/Source/core/input/MouseEventManager.cpp
diff --git a/third_party/WebKit/Source/core/input/MouseEventManager.cpp b/third_party/WebKit/Source/core/input/MouseEventManager.cpp
index aaca3de4a6922a173bbbca8e925282d38145c232..e9b88ef88231af4c310d7bec7a7d5aab6a520ba0 100644
--- a/third_party/WebKit/Source/core/input/MouseEventManager.cpp
+++ b/third_party/WebKit/Source/core/input/MouseEventManager.cpp
@@ -897,8 +897,8 @@ WebInputEventResult MouseEventManager::dispatchDragEvent(
   DragEvent* me = DragEvent::create(
       eventType, true, cancelable, m_frame->document()->domWindow(), 0,
       event.globalX, event.globalY, position.x(), position.y(), movement.x(),
-      movement.y(), static_cast<PlatformEvent::Modifiers>(event.modifiers()), 0,
-      MouseEvent::platformModifiersToButtons(event.modifiers()), nullptr,
+      movement.y(), static_cast<WebInputEvent::Modifiers>(event.modifiers()), 0,
+      MouseEvent::webInputEventModifiersToButtons(event.modifiers()), nullptr,
       TimeTicks::FromSeconds(event.timeStampSeconds()), dataTransfer,
       event.fromTouch() ? MouseEvent::FromTouch
                         : MouseEvent::RealOrIndistinguishable);