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

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

 /*
  * Copyright (C) 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 20 matching lines @@
 #define TextTrackContainer_h
 
 #include "core/html/HTMLDivElement.h"
 
 namespace blink {
 
 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
   // layout when the media controls become visible, to avoid overlapping them.
   enum ExposingControls {
     kDidNotStartExposingControls,
     kDidStartExposingControls
   };
   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> video_size_observer_;
+  float default_font_size_;
 };
 
 DEFINE_ELEMENT_TYPE_CASTS(TextTrackContainer, IsTextTrackContainer());
 
 }  // namespace blink
 
 #endif  // TextTrackContainer_h