Fix a FIXME - correctly calculate the main size when writing modes are involved.

Source/core/layout/LayoutFlexibleBox.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 392 matching lines @@
         computeLogicalHeight(borderBoxLogicalHeight, logicalTop(), computedValues);
         if (computedValues.m_extent == LayoutUnit::max())
             return computedValues.m_extent;
         return std::max(LayoutUnit(0), computedValues.m_extent - borderPaddingAndScrollbar);
     }
     return contentLogicalWidth();
 }
 
 LayoutUnit LayoutFlexibleBox::computeMainAxisExtentForChild(LayoutBox& child, SizeType sizeType, const Length& size)
 {
-    // FIXME: This is wrong for orthogonal flows. It should use the flexbox's writing-mode, not the child's in order
-    // to figure out the logical height/width.
-    if (isColumnFlow()) {
+    // If we have a horizontal flow, that means the main size is the width.
+    // That's the logical width for horizontal writing modes, and the logical height in vertical writing modes.
+    // For a vertical flow, main size is the height, so it's the inverse.
+    // So we need the logical width if we have a horizontal flow and horizontal writing mode, or vertical flow and vertical writing mode.
+    // Otherwise we need the logical height.
+    if (isHorizontalFlow() != child.styleRef().isHorizontalWritingMode()) {
         // We don't have to check for "auto" here - computeContentLogicalHeight will just return -1 for that case anyway.
         if (size.isIntrinsic())
             child.layoutIfNeeded();
         return child.computeContentLogicalHeight(size, child.logicalHeight() - child.borderAndPaddingLogicalHeight()) + child.scrollbarLogicalHeight();
     }
     return child.computeLogicalWidthUsing(sizeType, size, contentLogicalWidth(), this) - child.borderAndPaddingLogicalWidth();
 }
 
 WritingMode LayoutFlexibleBox::transformedWritingMode() const
 {
@@ skipping 972 matching lines @@
             ASSERT(child);
             LayoutUnit lineCrossAxisExtent = lineContexts[lineNumber].crossAxisExtent;
             LayoutUnit originalOffset = lineContexts[lineNumber].crossAxisOffset - crossAxisStartEdge;
             LayoutUnit newOffset = contentExtent - originalOffset - lineCrossAxisExtent;
             adjustAlignmentForChild(*child, newOffset - originalOffset);
         }
     }
 }
 
 }