[CSS Grid Layout] Flex tracks sizing alg must handle 0fr values

Source/core/css/parser/LegacyCSSPropertyParser.cpp

 /*
  * Copyright (C) 2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2012 Adobe Systems Incorporated. All rights reserved.
  * Copyright (C) 2012 Intel Corporation. All rights reserved.
  *
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 PassRefPtrWillBeRawPtr<CSSPrimitiveValue> CSSPropertyParser::parseGridBreadth(CSSParserValue* currentValue)
 {
     if (currentValue->id == CSSValueMinContent || currentValue->id == CSSValueMaxContent || currentValue->id == CSSValueAuto)
         return cssValuePool().createIdentifierValue(currentValue->id);
 
     if (currentValue->unit() == CSSPrimitiveValue::UnitType::Fraction) {
         double flexValue = currentValue->fValue;
 
         // Fractional unit is a non-negative dimension.
-        if (flexValue <= 0)
+        if (flexValue < 0)
             return nullptr;
 
         return cssValuePool().createValue(flexValue, CSSPrimitiveValue::UnitType::Fraction);
     }
 
     if (!validUnit(currentValue, FNonNeg | FLength | FPercent))
         return nullptr;
 
     return createPrimitiveNumericValue(currentValue);
 }
@@ skipping 4097 matching lines @@
         }
     }
 
     if (!list->length())
         return nullptr;
 
     return list.release();
 }
 
 } // namespace blink