Have Node::document() return a reference instead of a pointer

Source/core/rendering/FilterEffectRenderer.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * 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 252 matching lines @@
             effect = FEDropShadow::create(this, stdDeviation, stdDeviation, x, y, dropShadowOperation->color(), 1);
             break;
         }
         case FilterOperation::CUSTOM:
             // CUSTOM operations are always converted to VALIDATED_CUSTOM before getting here.
             // The conversion happens in RenderLayer::computeFilterOperations.
             ASSERT_NOT_REACHED();
             break;
         case FilterOperation::VALIDATED_CUSTOM: {
             ValidatedCustomFilterOperation* customFilterOperation = static_cast<ValidatedCustomFilterOperation*>(filterOperation);
-            Document* document = renderer ? renderer->document() : 0;
+            Document* document = renderer ? &renderer->document() : 0;
             effect = createCustomFilterEffect(this, document, customFilterOperation);
             if (effect)
                 m_hasCustomShaderFilter = true;
             break;
         }
         default:
             break;
         }
 
         if (effect) {
@@ skipping 152 matching lines @@
     // Get the filtered output and draw it in place.
     m_savedGraphicsContext->drawImageBuffer(filter->output(), filter->outputRect(), CompositeSourceOver);
 
     filter->clearIntermediateResults();
 
     return m_savedGraphicsContext;
 }
 
 } // namespace WebCore