Update assertion in LayoutBox::intrinsicScrollbarLogicalWidth.

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
  * modify it under the terms of the GNU Library General Public
@@ skipping 681 matching lines @@
 
     return layer()->scrollableArea()->horizontalScrollbarHeight();
 }
 
 int LayoutBox::intrinsicScrollbarLogicalWidth() const
 {
     if (!hasOverflowClip())
         return 0;
 
     if (isHorizontalWritingMode() && style()->overflowY() == OSCROLL) {
-        ASSERT(layer()->scrollableArea() && layer()->scrollableArea()->hasVerticalScrollbar());
-        return verticalScrollbarWidth();
+        ASSERT(layer()->scrollableArea());
+        // Even with OSCROLL, the scrollbar may not exist (crbug.com/415031).
+        return layer()->scrollableArea()->hasVerticalScrollbar() ? verticalScrollbarWidth() : 0;
     }
 
     if (!isHorizontalWritingMode() && style()->overflowX() == OSCROLL) {
-        ASSERT(layer()->scrollableArea() && layer()->scrollableArea()->hasHorizontalScrollbar());
-        return horizontalScrollbarHeight();
+        ASSERT(layer()->scrollableArea());
+        // Even with OSCROLL, the scrollbar may not exist (crbug.com/415031).
+        return layer()->scrollableArea()->hasHorizontalScrollbar() ? horizontalScrollbarHeight() : 0;
     }
 
     return 0;
 }
 
 ScrollResultOneDimensional LayoutBox::scroll(ScrollDirectionPhysical direction, ScrollGranularity granularity, float delta)
 {
     // Presumably the same issue as in setScrollTop. See crbug.com/343132.
     DisableCompositingQueryAsserts disabler;
 
@@ skipping 4015 matching lines @@
     StyleImage* borderImage = style()->borderImage().image();
     return borderImage && borderImage->canRender(*this, style()->effectiveZoom()) && borderImage->isLoaded();
 }
 
 ShapeOutsideInfo* LayoutBox::shapeOutsideInfo() const
 {
     return ShapeOutsideInfo::isEnabledFor(*this) ? ShapeOutsideInfo::info(*this) : nullptr;
 }
 
 } // namespace blink