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 4f51a224bf1d6b84d7806be7f4f0ebbd5f5a7d20..874a250f152686f06f4255b646683cf7f3f79962 100644
--- a/third_party/WebKit/Source/core/input/MouseEventManager.cpp
+++ b/third_party/WebKit/Source/core/input/MouseEventManager.cpp
@@ -895,8 +895,8 @@ 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);