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

Source/core/css/CSSComputedStyleDeclaration.cpp

Index: Source/core/css/CSSComputedStyleDeclaration.cpp
diff --git a/Source/core/css/CSSComputedStyleDeclaration.cpp b/Source/core/css/CSSComputedStyleDeclaration.cpp
index f1534a5654d0d71728f00f0b97dee1260915e376..7bb9d6cd4285bfa8494fb201343ca32319ea6518 100644
--- a/Source/core/css/CSSComputedStyleDeclaration.cpp
+++ b/Source/core/css/CSSComputedStyleDeclaration.cpp
@@ -1548,6 +1548,22 @@ static ItemPosition resolveAlignmentAuto(ItemPosition position, Node* element)
     return isFlexOrGrid ? ItemPositionStretch : ItemPositionStart;
 }
 
+static void resolveAlignmentContentAuto(RenderStyle& style)
+{
+    if (style.justifyContent() != ContentPositionAuto || style.justifyContentDistribution() != ContentDistributionDefault)
+        return;
+
+    // Block Containers: For table cells, the behavior of the 'auto' depends on the computed
+    // value of 'vertical-align', otherwise behaves as 'start'.
+    // Flex Containers: 'auto' computes to 'stretch'.
+    // Grid Containers: 'auto' computes to 'start', and 'stretch' behaves like 'start'.
+    if (style.isDisplayFlexibleBox()) {
+        style.setJustifyContent(ContentPositionFlexStart);
+    } else {
+        style.setJustifyContent(ContentPositionStart);
+    }
+}
+
 static PassRefPtrWillBeRawPtr<CSSValueList> valueForItemPositionWithOverflowAlignment(ItemPosition itemPosition, OverflowAlignment overflowAlignment, ItemPositionType positionType)
 {
     RefPtrWillBeRawPtr<CSSValueList> result = CSSValueList::createSpaceSeparated();
@@ -1560,6 +1576,20 @@ static PassRefPtrWillBeRawPtr<CSSValueList> valueForItemPositionWithOverflowAlig
     return result.release();
 }
 
+static PassRefPtrWillBeRawPtr<CSSValueList> valueForContentPositionAndDistributionWithOverflowAlignment(ContentPosition position, OverflowAlignment overflowAlignment, ContentDistributionType distribution)
+{
+    RefPtrWillBeRawPtr<CSSValueList> result = CSSValueList::createSpaceSeparated();
+    if (distribution != ContentDistributionDefault)
+        result->append(CSSPrimitiveValue::create(distribution));
+    if (distribution == ContentDistributionDefault || position != ContentPositionAuto)
+        result->append(CSSPrimitiveValue::create(position));
+    if ((position >= ContentPositionCenter || distribution != ContentDistributionDefault) && overflowAlignment != OverflowAlignmentDefault)
+        result->append(CSSPrimitiveValue::create(overflowAlignment));
+    ASSERT(result->length() > 0);
+    ASSERT(result->length() <= 3);
+    return result.release();
+}
+
 PassRefPtrWillBeRawPtr<CSSValue> CSSComputedStyleDeclaration::getPropertyCSSValue(CSSPropertyID propertyID, EUpdateLayout updateLayout) const
 {
     Node* styledNode = this->styledNode();
@@ -1847,7 +1877,8 @@ PassRefPtrWillBeRawPtr<CSSValue> CSSComputedStyleDeclaration::getPropertyCSSValu
         case CSSPropertyFlexWrap:
             return cssValuePool().createValue(style->flexWrap());
         case CSSPropertyJustifyContent:
-            return cssValuePool().createValue(style->justifyContent());
+            resolveAlignmentContentAuto(*style);
+            return valueForContentPositionAndDistributionWithOverflowAlignment(style->justifyContent(), style->justifyContentOverflowAlignment(), style->justifyContentDistribution());
         case CSSPropertyOrder:
             return cssValuePool().createValue(style->order(), CSSPrimitiveValue::CSS_NUMBER);
         case CSSPropertyFloat: