Make RenderObject::style() return a const object

Source/core/css/resolver/ViewportStyleResolver.cpp

 /*
  * Copyright (C) 2012-2013 Intel Corporation. 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.
@@ skipping 161 matching lines @@
 
     RefPtrWillBeRawPtr<CSSValue> value = m_propertySet->getPropertyCSSValue(id);
     if (!value || !value->isPrimitiveValue())
         return Length(); // auto
 
     CSSPrimitiveValue* primitiveValue = toCSSPrimitiveValue(value.get());
 
     if (primitiveValue->getValueID() == CSSValueInternalExtendToZoom)
         return Length(ExtendToZoom);
 
-    RenderStyle* documentStyle = m_document->renderStyle();
+    const RenderStyle* documentStyle = m_document->renderStyle();
 
     // If we have viewport units the conversion will mark the document style as having viewport units.
     bool documentStyleHasViewportUnits = documentStyle->hasViewportUnits();
     documentStyle->setHasViewportUnits(false);
 
     FrameView* view = m_document->view();
     float width = view ? view->width() : 0;
     float height = view ? view->height() : 0;
 
     CSSToLengthConversionData::FontSizes fontSizes(documentStyle, documentStyle);
     CSSToLengthConversionData::ViewportSize viewportSize(width, height);
     Length result = primitiveValue->convertToLength<AnyConversion>(CSSToLengthConversionData(documentStyle, fontSizes, viewportSize, 1.0f));
     if (documentStyle->hasViewportUnits())
         m_document->setHasViewportUnits();
     documentStyle->setHasViewportUnits(documentStyleHasViewportUnits);
 
     return result;
 }
 
 void ViewportStyleResolver::trace(Visitor* visitor)
 {
     visitor->trace(m_propertySet);
     visitor->trace(m_document);
 }
 
 } // namespace blink