PlzNavigate: Allow frames to fallback to alternative content.

third_party/WebKit/Source/core/html/HTMLFrameOwnerElement.h

 /*
  * Copyright (C) 2006, 2007, 2009 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 62 matching lines @@
 
    private:
     void performDeferredWidgetTreeOperations();
   };
 
   // FrameOwner overrides:
   void setContentFrame(Frame&) override;
   void clearContentFrame() override;
   void dispatchLoad() override;
   SandboxFlags getSandboxFlags() const override { return m_sandboxFlags; }
+  bool canRenderFallbackContent() const override { return false; }
   void renderFallbackContent() override {}
   ScrollbarMode scrollingMode() const override { return ScrollbarAuto; }
   int marginWidth() const override { return -1; }
   int marginHeight() const override { return -1; }
   bool allowFullscreen() const override { return false; }
   AtomicString csp() const override { return nullAtom; }
   const WebVector<WebPermissionType>& delegatedPermissions() const override;
 
   DECLARE_VIRTUAL_TRACE();
 
@@ skipping 62 matching lines @@
 
 DEFINE_TYPE_CASTS(HTMLFrameOwnerElement,
                   FrameOwner,
                   owner,
                   owner->isLocal(),
                   owner.isLocal());
 
 }  // namespace blink
 
 #endif  // HTMLFrameOwnerElement_h