[CSS Grid Layout] overflow-position keyword for align and justify properties.

Source/core/css/resolver/StyleAdjuster.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 232 matching lines @@
     }
 
     adjustStyleForAlignment(*style, *parentStyle);
 }
 
 void StyleAdjuster::adjustStyleForAlignment(RenderStyle& style, const RenderStyle& parentStyle)
 {
     bool isFlexOrGrid = style.isDisplayFlexibleOrGridBox();
     bool absolutePositioned = style.position() == AbsolutePosition;
 
+    // The default overflow alignment is 'true' for flex or grid items/contaners, 'safe' otherwise.
+    if (style.justifyItemsOverflowAlignment() == OverflowAlignmentDefault)
+        style.setJustifyItemsOverflowAlignment(isFlexOrGrid ? OverflowAlignmentTrue : OverflowAlignmentSafe);
+    if (style.justifySelfOverflowAlignment() == OverflowAlignmentDefault)
+        style.setJustifySelfOverflowAlignment(isFlexOrGrid ? OverflowAlignmentTrue : OverflowAlignmentSafe);
+
     // If the inherited value of justify-items includes the legacy keyword, 'auto'
     // computes to the the inherited value.
     // Otherwise, auto computes to:
     //  - 'stretch' for flex containers and grid containers.
     //  - 'start' for everything else.
     if (style.justifyItems() == ItemPositionAuto) {
         if (parentStyle.justifyItemsPositionType() == LegacyPosition) {
             style.setJustifyItems(parentStyle.justifyItems());
             style.setJustifyItemsPositionType(parentStyle.justifyItemsPositionType());
         } else if (isFlexOrGrid) {
@@ skipping 186 matching lines @@
     if (style->writingMode() != TopToBottomWritingMode && (style->display() == BOX || style->display() == INLINE_BOX))
         style->setWritingMode(TopToBottomWritingMode);
 
     if (parentStyle->isDisplayFlexibleOrGridBox()) {
         style->setFloating(NoFloat);
         style->setDisplay(equivalentBlockDisplay(style->display(), style->isFloating(), !m_useQuirksModeStyles));
     }
 }
 
 }