Don't always have to relayout a child when fragmentainer height is unknown.

third_party/WebKit/Source/core/layout/LayoutBox.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc.
  *               All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 4724 matching lines @@
     if (child.offsetToNextPage()) {
       // We need to relayout unless we're going to break at the exact same
       // location as before.
       if (child.offsetToNextPage() != remainingSpace)
         return true;
     } else if (logicalHeight > remainingSpace) {
       // Last time we laid out this child, we didn't need to break, but now we
       // have to. So we need to relayout.
       return true;
     }
-  } else {
+  } else if (child.offsetToNextPage()) {
+    // This child did previously break, but it won't anymore, because we no
+    // longer have a known fragmentainer height.
     return true;
   }
 
   // It seems that we can skip layout of this child, but we need to ask the flow
   // thread for permission first. We currently cannot skip over objects
   // containing column spanners.
   LayoutFlowThread* flowThread = child.flowThreadContainingBlock();
   return flowThread && !flowThread->canSkipLayout(child);
 }
 
@@ skipping 882 matching lines @@
   LayoutRect rect = frameRect();
 
   LayoutBlock* block = containingBlock();
   if (block)
     block->adjustChildDebugRect(rect);
 
   return rect;
 }
 
 }  // namespace blink