Allow flow thread portion logical bottom to be above its logical top.

third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.h

Index: third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.h b/third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.h
index d92d9735c62f0033389ae67afca5a111720a167c..33ceea3f10d766f47cb68c0a94571aa128332b0d 100644
--- a/third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.h
+++ b/third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.h
@@ -149,7 +149,8 @@ class CORE_EXPORT LayoutMultiColumnSet : public LayoutBlockFlow {
   LayoutUnit logicalTopInFlowThread() const;
   LayoutUnit logicalBottomInFlowThread() const;
   LayoutUnit logicalHeightInFlowThread() const {

[eae, 2017/02/23 22:20:20]

Would you mind adding a comment here explaining how this could be negative (i.e.
if the top is above the bottom). For most casual readers that would seem to be
impossible thus making this check seem unnecessary. 

[mstensho (USE GERRIT), 2017/02/27 11:57:05]

Good idea. Done.

-    return logicalBottomInFlowThread() - logicalTopInFlowThread();
+    return (logicalBottomInFlowThread() - logicalTopInFlowThread())
+        .clampNegativeToZero();
   }
 
   // Return the amount of flow thread contents that the specified fragmentainer