Support subtitles for native fullscreen video

Source/core/rendering/RenderLayerModelObject.h

Index: Source/core/rendering/RenderLayerModelObject.h
diff --git a/Source/core/rendering/RenderLayerModelObject.h b/Source/core/rendering/RenderLayerModelObject.h
index 959853bf4fa885cc557e29ac45f74d425203152e..414fe361faa3c606da48f1395e64930fd1cc3bfa 100644
--- a/Source/core/rendering/RenderLayerModelObject.h
+++ b/Source/core/rendering/RenderLayerModelObject.h
@@ -28,6 +28,7 @@
 namespace WebCore {
 
 class RenderLayer;
+class RenderLayerBacking;
 class ScrollableArea;
 
 class RenderLayerModelObject : public RenderObject {
@@ -55,6 +56,8 @@ public:
     // This is null for anonymous renderers.
     ContainerNode* node() const { return toContainerNode(RenderObject::node()); }
 
+    RenderLayerBacking* backing() const;
+
 protected:
     void ensureLayer();