Restore sanity to RenderLayerStackingNode::shouldBeNormalFlowOnly

Source/core/rendering/RenderLayerStackingNode.cpp

Index: Source/core/rendering/RenderLayerStackingNode.cpp
diff --git a/Source/core/rendering/RenderLayerStackingNode.cpp b/Source/core/rendering/RenderLayerStackingNode.cpp
index a750ce8b08d10930d070635fcb7fe0bce1bb8bb3..a6ad456c69f5c955a97a3415e2faf3f584441177 100644
--- a/Source/core/rendering/RenderLayerStackingNode.cpp
+++ b/Source/core/rendering/RenderLayerStackingNode.cpp
@@ -308,25 +308,7 @@ void RenderLayerStackingNode::updateStackingNodesAfterStyleChange(const RenderSt
 
 bool RenderLayerStackingNode::shouldBeNormalFlowOnly() const
 {
-    RenderLayerModelObject* renderer = this->renderer();
-
-    const bool couldBeNormalFlow = renderer->hasOverflowClip()
-        || renderer->hasReflection()
-        || renderer->hasMask()
-        || renderer->isCanvas()
-        || renderer->isVideo()
-        || renderer->isEmbeddedObject()
-        || renderer->isRenderIFrame()
-        || (renderer->style()->specifiesColumns() && !layer()->isRootLayer());
-
-    const bool preventsElementFromBeingNormalFlow = renderer->isPositioned()
-        || renderer->hasTransform()
-        || renderer->hasClipPath()
-        || renderer->hasFilter()
-        || renderer->hasBlendMode()
-        || layer()->isTransparent();
-
-    return couldBeNormalFlow && !preventsElementFromBeingNormalFlow;
+    return !isStackingContext() && !renderer()->isPositioned();

[Julien - ping for review, 2014/06/03 00:53:20]

Based on the comments in the file, it really looks like "normal flow" just means
overflow-only layer (which is close to being a non-self-painting-layer). We
should amend these comments else it's going to be confusing :-)

Now CSS 2.1 defines normal flow as not absolutely / fixed positioned and not
floating. I think we should use this definition here so:
- it's missing a isFloating() check.
- isPositioned() is basically position != static so it should be
isOutOfFlowPositioned() instead.
- Is the stackingContext() call needed? If so, I would like a comment.

 }
 
 void RenderLayerStackingNode::updateIsNormalFlowOnly()