[CSS Grid Layout] Stretch value for align and justify properties.

Source/core/rendering/style/RenderStyle.h

Index: Source/core/rendering/style/RenderStyle.h
diff --git a/Source/core/rendering/style/RenderStyle.h b/Source/core/rendering/style/RenderStyle.h
index b63388337df13da2fb11621997573a2c382b9530..eb4fcd39525fe1f5c1007c68e472cc849fad6627 100644
--- a/Source/core/rendering/style/RenderStyle.h
+++ b/Source/core/rendering/style/RenderStyle.h
@@ -346,7 +346,8 @@ public:
     // Computes how the style change should be propagated down the tree.
     static StyleRecalcChange stylePropagationDiff(const RenderStyle* oldStyle, const RenderStyle* newStyle);
 
-    static ItemPosition resolveAlignment(const RenderStyle* parentStyle, const RenderStyle* childStyle);
+    static ItemPosition resolveAlignment(const RenderStyle* parentStyle, const RenderStyle* childStyle, ItemPosition defaultPosition = ItemPositionStretch);
+    static ItemPosition resolveJustification(const RenderStyle* parentStyle, const RenderStyle* childStyle, ItemPosition defaultPosition = ItemPositionStretch);

[Julien - ping for review, 2014/10/31 17:17:13]

The default parameter seems very artificial (why ItemPositionStretch when the
initial is ItemPositionStart? Also the spec has no real 'default' for this
value), it would be better without a default.

Also let's rename |defaultPosition| to something else. Suggestions:
|resolvedAutoPositionForRenderer|, |positionForAutoBasedOnRenderer|

[jfernandez, 2014/10/31 22:55:59]

I agree is not the best approach, but I couldn't come out with anything better.
It ight be that it'd be better after all not to move this logic to RenderStyle
to be shared between Flexbox and Grid, for now, and other kinds of block.

The main problem is that the spec states that "auto' is resolved to 'stretch" 
for Flex and Grid containers, and 'start' otherwise. I've decided to initialize
the parameter to 'stretch' to avoid touching the only Renders which use the
alignment properties nowadays. Other renders should pass 'start' as value for
such argument.

The thing is that we have had solved the 'auto' resolution during the cascade
(StyleAdjuster), at least for align-items/justify-items, and for the computed
value. However, there are cases where the RenderTree is modified after the
cascade, new elements introduced or updated, so the alignment properties might
be 'auto' anyway.  Having the 'auto' resolution at Rendering side was something
useful to ensure the correct value. 
I agree, though, that default may be not the best name. Any of your suggestions
are really better. I'll think about this for the final version of the patch to
be landed.

 
     StyleDifference visualInvalidationDiff(const RenderStyle&) const;