Isolate strictly unbreakable multicol containers that are nested.

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

Index: third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.h b/third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.h
index 8264eb4944e68bce1e292071b88e67b17e350c02..91fb0d701095c9232c04cb2d6a73b1d25d17f229 100644
--- a/third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.h
+++ b/third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.h
@@ -260,10 +260,25 @@ class CORE_EXPORT LayoutMultiColumnFlowThread : public LayoutFlowThread,
   bool removeSpannerPlaceholderIfNoLongerValid(
       LayoutBox* spannerObjectInFlowThread);
 
+  // Search mode when looking for an enclosing fragmentation context.
+  enum AncestorSearchConstraint {
+    // No constraints. Sometimes we just want to find all enclosing
+    // fragmentation contexts, e.g. to calculate the accumulated visual
+    // translation.
+    AnyAncestor,
+
+    // Consider fragmentation contexts that are strictly unbreakable (seen from
+    // the outside) to be isolated from the rest, so that such fragmentation
+    // contexts don't participate in fragmentation of enclosing fragmentation
+    // contexts, apart from taking up space and otherwise being completely
+    // unbreakable. This is typically what we want to do during layout.
+    IsolateUnbreakableContainers,
+  };
   LayoutMultiColumnFlowThread* enclosingFlowThread() const;
-  FragmentationContext* enclosingFragmentationContext() const;
+  FragmentationContext* enclosingFragmentationContext(
+      AncestorSearchConstraint = IsolateUnbreakableContainers) const;
   LayoutUnit blockOffsetInEnclosingFragmentationContext() const {
-    ASSERT(enclosingFragmentationContext());
+    DCHECK(enclosingFragmentationContext(AnyAncestor));
     return m_blockOffsetInEnclosingFragmentationContext;
   }