Dispel the myth that enclosingLayer can return zero

Source/core/events/MouseRelatedEvent.cpp

 /*
  * 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, 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 142 matching lines @@
     }
 
     // Adjust layerLocation to be relative to the layer.
     // FIXME: event.layerX and event.layerY are poorly defined,
     // and probably don't always correspond to RenderLayer offsets.
     // https://bugs.webkit.org/show_bug.cgi?id=21868
     Node* n = targetNode;
     while (n && !n->renderer())
         n = n->parentNode();
 
-    RenderLayer* layer;
-    if (n && (layer = n->renderer()->enclosingLayer())) {
-        for (; layer; layer = layer->parent()) {
+    if (n) {
+        for (RenderLayer* layer = n->renderer()->enclosingLayer(); layer; layer = layer->parent())
             m_layerLocation -= toLayoutSize(layer->location());
-        }
     }
 
     m_hasCachedRelativePosition = true;
 }
 
 int MouseRelatedEvent::layerX()
 {
     if (!m_hasCachedRelativePosition)
         computeRelativePosition();
     return m_layerLocation.x();
@@ skipping 43 matching lines @@
     // See Microsoft documentation and <http://www.quirksmode.org/dom/w3c_events.html>.
     return m_clientLocation.y();
 }
 
 void MouseRelatedEvent::trace(Visitor* visitor)
 {
     UIEventWithKeyState::trace(visitor);
 }
 
 } // namespace WebCore