Propagate writing-mode from the first region to the flow thread.

Source/core/rendering/RenderFlowThread.h

Index: Source/core/rendering/RenderFlowThread.h
diff --git a/Source/core/rendering/RenderFlowThread.h b/Source/core/rendering/RenderFlowThread.h
index 18fb7db4857cbdf216d255c80a97e4034f57cb3f..3da1907337f4f7716a89fa4615d6455f70b18e03 100644
--- a/Source/core/rendering/RenderFlowThread.h
+++ b/Source/core/rendering/RenderFlowThread.h
@@ -87,6 +87,7 @@ public:
     // Check if the content is flown into at least a region with region styling rules.
     bool hasRegionsWithStyling() const { return m_hasRegionsWithStyling; }
     void checkRegionsWithStyling();
+    virtual void regionChangedWritingMode(RenderRegion*) { }
 
     void validateRegions();
     void invalidateRegions();