[CSS Grid Layout] Upgrade justify-content parsing to CSS3 Box Alignment spec.

Source/core/css/CSSPrimitiveValue.h

Index: Source/core/css/CSSPrimitiveValue.h
diff --git a/Source/core/css/CSSPrimitiveValue.h b/Source/core/css/CSSPrimitiveValue.h
index 242fcda4eb610f028626fa919fdf1d52839be092..06b08e928932905d578bff457ea807867b31a881 100644
--- a/Source/core/css/CSSPrimitiveValue.h
+++ b/Source/core/css/CSSPrimitiveValue.h
@@ -213,6 +213,19 @@ public:
     bool isValueID() const { return m_primitiveUnitType == CSS_VALUE_ID; }
     bool colorIsDerivedFromElement() const;
 
+    static bool isContentDistributionKeyword(CSSValueID id)
+    {
+        return id == CSSValueSpaceBetween || id == CSSValueSpaceAround
+            || id == CSSValueSpaceEvenly || id == CSSValueStretch;
+    }
+
+    static bool isContentPositionKeyword(CSSValueID id)
+    {
+        return id == CSSValueStart || id == CSSValueEnd || id == CSSValueCenter
+            || id == CSSValueFlexStart || id == CSSValueFlexEnd
+            || id == CSSValueLeft || id == CSSValueRight;
+    }
+
     static PassRefPtrWillBeRawPtr<CSSPrimitiveValue> createIdentifier(CSSValueID valueID)
     {
         return adoptRefWillBeNoop(new CSSPrimitiveValue(valueID));