Don't allow position:sticky for table columns.

Source/core/rendering/RenderTableCol.h

Index: Source/core/rendering/RenderTableCol.h
diff --git a/Source/core/rendering/RenderTableCol.h b/Source/core/rendering/RenderTableCol.h
index f3ccef57aa1181b5e7a8465f2c8d672b81f0612e..c5c2882c635d23d2ff0c9a123112b8484b896375 100644
--- a/Source/core/rendering/RenderTableCol.h
+++ b/Source/core/rendering/RenderTableCol.h
@@ -90,7 +90,13 @@ private:
 
     virtual bool isChildAllowed(RenderObject*, RenderStyle*) const OVERRIDE;
     virtual bool canHaveChildren() const OVERRIDE;
-    virtual LayerType layerTypeRequired() const OVERRIDE { return NoLayer; }
+    virtual LayerType layerTypeRequired() const OVERRIDE
+    {
+        if (isStickyPositioned())

[Julien - ping for review, 2014/03/12 03:14:42]

columns are only style placeholder. That's the reason we never allocate a layer
for them. Could you explain why you feel this is the right fix (I may be missing
some context here)?

[ostap, 2014/03/12 05:26:24]

I think that table column can be sticky positioned. Is it right?

[Julien - ping for review, 2014/03/12 15:17:04]

Yes, it can but that doesn't mean that we should allow it! It wouldn't be the
first time we adjust the style to prevent some situation we don't want from
happening (see StyleAdjuster).

Back on the current change, I was asking why you took this approach because for
me it doesn't feel like the right fix. We disable RenderLayers on <col> because
they make no sense. On top of that, we don't have a solid answer whether sticky
requires a RenderLayer (e.g. position:relative doesn't)

[ostap, 2014/03/12 15:51:12]

Ok. I'll disable sticky support for table elements that don't create
RenderLayer.
I think because relative positioned elements don't change their offset
dynamically, but sticky do change based on scroll position of the related
scrollbox.

+            return NormalLayer;
+
+        return NoLayer;
+    }
 
     virtual LayoutRect clippedOverflowRectForRepaint(const RenderLayerModelObject* repaintContainer) const OVERRIDE;
     virtual void imageChanged(WrappedImagePtr, const IntRect* = 0) OVERRIDE;