Use correct container width as base for percentage margins.

Source/core/rendering/RenderBox.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2006, 2007 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 371 matching lines @@
     };
     struct LogicalExtentComputedValues {
         LogicalExtentComputedValues() { }
 
         LayoutUnit m_extent;
         LayoutUnit m_position;
         ComputedMarginValues m_margins;
     };
     // Resolve auto margins in the inline direction of the containing block so that objects can be pushed to the start, middle or end
     // of the containing block.
-    void computeInlineDirectionMargins(RenderBlock* containingBlock, LayoutUnit containerWidth, LayoutUnit childWidth, LayoutUnit& marginStart, LayoutUnit& marginEnd) const;
+    void computeInlineDirectionMargins(RenderBlock* containingBlock, LayoutUnit containerWidth, LayoutUnit availableWidth, LayoutUnit childWidth, LayoutUnit& marginStart, LayoutUnit& marginEnd) const;
 
     // Used to resolve margins in the containing block's block-flow direction.
     void computeBlockDirectionMargins(const RenderBlock* containingBlock, LayoutUnit& marginBefore, LayoutUnit& marginAfter) const;
     void computeAndSetBlockDirectionMargins(const RenderBlock* containingBlock);
 
     virtual LayoutUnit offsetFromLogicalTopOfFirstPage() const;
 
     void positionLineBox(InlineBox*);
 
     virtual InlineBox* createInlineBox();
@@ skipping 371 matching lines @@
         if (UNLIKELY(inlineBoxWrapper() != 0))
             deleteLineBoxWrapper();
     }
 
     ensureRareData().m_inlineBoxWrapper = boxWrapper;
 }
 
 } // namespace WebCore
 
 #endif // RenderBox_h