Remove useless null checks from Node::document() call sites

Source/core/page/FocusController.cpp

 /*
  * Copyright (C) 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nuanti Ltd.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 770 matching lines @@
         if (candidate.isNull())
             continue;
 
         candidate.enclosingScrollableBox = container;
         updateFocusCandidateIfNeeded(direction, current, candidate, closest);
     }
 }
 
 bool FocusController::advanceFocusDirectionallyInContainer(Node* container, const LayoutRect& startingRect, FocusDirection direction)
 {
-    if (!container || !container->document())
+    if (!container)
         return false;
 
     LayoutRect newStartingRect = startingRect;
 
     if (startingRect.isEmpty())
         newStartingRect = virtualRectForDirection(direction, nodeRectInAbsoluteCoordinates(container));
 
     // Find the closest node within current container in the direction of the navigation.
     FocusCandidate focusCandidate;
     findFocusCandidateInContainer(container, newStartingRect, direction, focusCandidate);
@@ skipping 88 matching lines @@
         startingRect = nodeRectInAbsoluteCoordinates(container, true /* ignore border */);
         container = scrollableEnclosingBoxOrParentFrameForNodeInDirection(direction, container);
         if (container && container->isDocumentNode())
             toDocument(container)->updateLayoutIgnorePendingStylesheets();
     } while (!consumed && container);
 
     return consumed;
 }
 
 } // namespace WebCore