Don't allow negative width for positioned elements

Source/core/rendering/RenderBox.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. 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
@@ skipping 3277 matching lines @@
         } else if (logicalLeftIsAuto && !logicalWidthIsAuto && !logicalRightIsAuto) {
             // RULE 4: (solve for left)
             computedValues.m_extent = adjustContentBoxLogicalWidthForBoxSizing(valueForLength(logicalWidth, containerLogicalWidth));
             logicalLeftValue = availableSpace - (computedValues.m_extent + valueForLength(logicalRight, containerLogicalWidth));
         } else if (!logicalLeftIsAuto && logicalWidthIsAuto && !logicalRightIsAuto) {
             // RULE 5: (solve for width)
             logicalLeftValue = valueForLength(logicalLeft, containerLogicalWidth);
             if (autoWidthShouldFitContent())
                 shrinkToFitWidth(availableSpace, logicalLeftValue, bordersPlusPadding, computedValues);
             else
-                computedValues.m_extent = availableSpace - (logicalLeftValue + valueForLength(logicalRight, containerLogicalWidth));
+                computedValues.m_extent = std::max<LayoutUnit>(0, availableSpace - (logicalLeftValue + valueForLength(logicalRight, containerLogicalWidth)));
         } else if (!logicalLeftIsAuto && !logicalWidthIsAuto && logicalRightIsAuto) {
             // RULE 6: (no need solve for right)
             logicalLeftValue = valueForLength(logicalLeft, containerLogicalWidth);
             computedValues.m_extent = adjustContentBoxLogicalWidthForBoxSizing(valueForLength(logicalWidth, containerLogicalWidth));
         }
     }
 
     // Use computed values to calculate the horizontal position.
 
     // FIXME: This hack is needed to calculate the  logical left position for a 'rtl' relatively
@@ skipping 1426 matching lines @@
 RenderBox::BoxDecorationData::BoxDecorationData(const RenderStyle& style)
 {
     backgroundColor = style.visitedDependentColor(CSSPropertyBackgroundColor);
     hasBackground = backgroundColor.alpha() || style.hasBackgroundImage();
     ASSERT(hasBackground == style.hasBackground());
     hasBorder = style.hasBorder();
     hasAppearance = style.hasAppearance();
 }
 
 } // namespace blink