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

Source/core/rendering/RenderNamedFlowThread.h

Index: Source/core/rendering/RenderNamedFlowThread.h
diff --git a/Source/core/rendering/RenderNamedFlowThread.h b/Source/core/rendering/RenderNamedFlowThread.h
index 1e41f991f84a6aedad7fb18dc0d5ac2bc4384294..c0dec6cbdc1ed64e0414f0a301e7ed50e6273888 100644
--- a/Source/core/rendering/RenderNamedFlowThread.h
+++ b/Source/core/rendering/RenderNamedFlowThread.h
@@ -68,6 +68,8 @@ public:
     virtual void addRegionToThread(RenderRegion*) OVERRIDE;
     virtual void removeRegionFromThread(RenderRegion*) OVERRIDE;
 
+    virtual void regionChangedWritingMode(RenderRegion*) OVERRIDE;
+
     bool overset() const { return m_overset; }
     void computeOversetStateForRegions(LayoutUnit oldClientAfterEdge);
 
@@ -104,6 +106,7 @@ private:
     void regionLayoutUpdateEventTimerFired(Timer<RenderNamedFlowThread>*);
     void regionOversetChangeEventTimerFired(Timer<RenderNamedFlowThread>*);
     void clearContentNodes();
+    void updateWritingMode();
 
 private:
     // Observer flow threads have invalid regions that depend on the state of this thread