Support subtitles for native fullscreen video

Source/core/rendering/RenderObject.h

Index: Source/core/rendering/RenderObject.h
diff --git a/Source/core/rendering/RenderObject.h b/Source/core/rendering/RenderObject.h
index 272d7657e7c429d2b41a3f8a074572588afa5d62..9880efd6ccb2fd3658557d53d290b01c58fbe198 100644
--- a/Source/core/rendering/RenderObject.h
+++ b/Source/core/rendering/RenderObject.h
@@ -244,6 +244,8 @@ public:
         for (const RenderObject* renderer = this; renderer; renderer = renderer->nextInPreOrder())
             renderer->assertRendererLaidOut();
     }
+
+    void setIsCurrentFullscreen(bool flag) { m_isCurrentFullscreen = flag; }
 #endif
 
     // Obtains the nearest enclosing block (including this block) that contributes a first-line style to our inline
@@ -1066,6 +1068,7 @@ private:
 #ifndef NDEBUG
     unsigned m_hasAXObject             : 1;
     unsigned m_setNeedsLayoutForbidden : 1;
+    unsigned m_isCurrentFullscreen     : 1;
 #endif
 
 #define ADD_BOOLEAN_BITFIELD(name, Name) \