Direct composite canvas background if possible.

Source/core/rendering/RenderVideo.h

Index: Source/core/rendering/RenderVideo.h
diff --git a/Source/core/rendering/RenderVideo.h b/Source/core/rendering/RenderVideo.h
index ee7168348ea352a7757635e539758c74bbb43700..53b73804945a0da99ed3259c7e5c01152409e8a1 100644
--- a/Source/core/rendering/RenderVideo.h
+++ b/Source/core/rendering/RenderVideo.h
@@ -85,6 +85,12 @@ inline RenderVideo* toRenderVideo(RenderObject* object)
     return static_cast<RenderVideo*>(object);
 }
 
+inline const RenderVideo* toRenderVideo(const RenderObject* object)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!object || object->isVideo());
+    return static_cast<const RenderVideo*>(object);
+}
+
 // This will catch anyone doing an unnecessary cast.
 void toRenderVideo(const RenderVideo*);