[New Multicolumn] Handle overflow inside spanners correctly.

Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp

Index: Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
diff --git a/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp b/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
index b0555a5b2af6c9766a4a282382d2e39e16ee7c6e..5af88b9eddcb666fa6b02562c7e645725d3b49d7 100644
--- a/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
+++ b/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
@@ -55,6 +55,12 @@ void RenderMultiColumnSpannerPlaceholder::layout()
     // height as well, so that we take up the correct amount of space in the multicol container.
     updateLogicalHeight();
 
+    // Take the overflow from the spanner as well, so that it gets propagated to the multicol
+    // container and beyond.

[Julien - ping for review, 2015/01/20 14:07:38]

Nit: Let's balance the comment better over the 2 lines.

[mstensho (USE GERRIT), 2015/01/20 16:31:56]

This was what I got from emacs. :-P

Done.

+    m_overflow.clear();
+    addVisualOverflow(m_rendererInFlowThread->visualOverflowRect());
+    addLayoutOverflow(m_rendererInFlowThread->layoutOverflowRect());

[Julien - ping for review, 2015/01/20 14:07:38]

I would recommend moving this code to an override of computeOverflow() and call
it here.

[mstensho (USE GERRIT), 2015/01/20 16:31:56]

Contrary to what I actually think I told you yesterday, that the spanner
placeholder is a RenderBlockFlow, it's *not*. Actually it's just a RenderBox
(because it doesn't need to be more than that). So I can't do that, since
computeOverflow() needs RenderBlock or better.

So I can't do this.

+
     clearNeedsLayout();
 }