Move renderFallbackContent() into FrameOwner interface.

Source/core/html/HTMLObjectElement.h

Index: Source/core/html/HTMLObjectElement.h
diff --git a/Source/core/html/HTMLObjectElement.h b/Source/core/html/HTMLObjectElement.h
index 81d2b892aca117ee28a631871269d0cbaa8f383a..0bb97508b7b3af346cf89871c24e10abf2bbe12a 100644
--- a/Source/core/html/HTMLObjectElement.h
+++ b/Source/core/html/HTMLObjectElement.h
@@ -55,8 +55,6 @@ public:
     virtual bool isInteractiveContent() const override;
     virtual bool appendFormData(FormDataList&, bool) override;
 
-    virtual bool isObjectElement() const override { return true; }
-
     // Implementations of constraint validation API.
     // Note that the object elements are always barred from constraint validation.
     virtual String validationMessage() const override { return String(); }