Remove unnecessary RenderFlowThread usage and inclusion.

Source/core/rendering/RenderBox.cpp

Index: Source/core/rendering/RenderBox.cpp
diff --git a/Source/core/rendering/RenderBox.cpp b/Source/core/rendering/RenderBox.cpp
index 9139f5819621ce5bd258eece12606525f26ecf53..348fd002c999a388fd5fe4bd7652946dd288f217 100644
--- a/Source/core/rendering/RenderBox.cpp
+++ b/Source/core/rendering/RenderBox.cpp
@@ -2555,7 +2555,7 @@ LayoutUnit RenderBox::availableLogicalHeightUsing(const Length& h, AvailableLogi
         return logicalHeight() - borderAndPaddingLogicalHeight();
     }
 
-    if (h.isPercent() && isOutOfFlowPositioned() && !isRenderFlowThread()) {
+    if (h.isPercent() && isOutOfFlowPositioned()) {
         // FIXME: This is wrong if the containingBlock has a perpendicular writing mode.
         LayoutUnit availableHeight = containingBlockLogicalHeightForPositioned(containingBlock());
         return adjustContentBoxLogicalHeightForBoxSizing(valueForLength(h, availableHeight));