Use a ResizeObserver to determine default font-size for text tracks

third_party/WebKit/Source/core/html/track/TextTrackContainer.h

Index: third_party/WebKit/Source/core/html/track/TextTrackContainer.h
diff --git a/third_party/WebKit/Source/core/html/track/TextTrackContainer.h b/third_party/WebKit/Source/core/html/track/TextTrackContainer.h
index 9f864c44941b5d72d450bde335b7e11773accd32..decf3a3df12a5f804e3a4896498bc1c2121f0092 100644
--- a/third_party/WebKit/Source/core/html/track/TextTrackContainer.h
+++ b/third_party/WebKit/Source/core/html/track/TextTrackContainer.h
@@ -38,7 +38,7 @@ class HTMLMediaElement;
 
 class TextTrackContainer final : public HTMLDivElement {
  public:
-  static TextTrackContainer* create(Document&);
+  static TextTrackContainer* create(HTMLMediaElement&);
 
   // Runs the "rules for updating the text track rendering". The
   // ExposingControls enum is used in the WebVTT processing model to reset the
@@ -48,13 +48,20 @@ class TextTrackContainer final : public HTMLDivElement {
     DidStartExposingControls
   };
   void updateDisplay(HTMLMediaElement&, ExposingControls);
+  void updateDefaultFontSize(LayoutObject*);
+
+  DECLARE_VIRTUAL_TRACE();
 
  private:
   TextTrackContainer(Document&);
 
   bool isTextTrackContainer() const override { return true; }
+  void observeSizeChanges(Element&);
 
   LayoutObject* createLayoutObject(const ComputedStyle&) override;
+
+  Member<ResizeObserver> m_videoSizeObserver;
+  float m_defaultFontSize;
 };
 
 DEFINE_ELEMENT_TYPE_CASTS(TextTrackContainer, isTextTrackContainer());