WebVTT: Support 'auto' text position

Source/core/html/track/vtt/VTTCue.h

Index: Source/core/html/track/vtt/VTTCue.h
diff --git a/Source/core/html/track/vtt/VTTCue.h b/Source/core/html/track/vtt/VTTCue.h
index 616783c69d507358cb555763fbe3fc36e4fb5e1e..01846418180b27d1b4858b0ae6cde6ef63692fd5 100644
--- a/Source/core/html/track/vtt/VTTCue.h
+++ b/Source/core/html/track/vtt/VTTCue.h
@@ -80,8 +80,8 @@ public:
     void line(DoubleOrAutoKeyword&) const;
     void setLine(const DoubleOrAutoKeyword&, ExceptionState&);
 
-    double position() const { return m_textPosition; }
-    void setPosition(double, ExceptionState&);
+    void position(DoubleOrAutoKeyword&) const;
+    void setPosition(const DoubleOrAutoKeyword&, ExceptionState&);
 
     double size() const { return m_cueSize; }
     void setSize(double, ExceptionState&);
@@ -160,6 +160,8 @@ private:
     bool lineIsAuto() const;
 
     void calculateDisplayParameters();
+    float calculateComputedTextPosition() const;
+    bool textPositionIsAuto() const;
 
     enum CueSetting {
         None,