PlzNavigate: Allow frames to fallback to alternative content.

third_party/WebKit/Source/core/frame/FrameOwner.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef FrameOwner_h
 #define FrameOwner_h
 
 #include "core/CoreExport.h"
 #include "core/dom/SandboxFlags.h"
 #include "platform/heap/Handle.h"
@@ skipping 17 matching lines @@
   virtual bool isRemote() const = 0;
 
   virtual void setContentFrame(Frame&) = 0;
   virtual void clearContentFrame() = 0;
 
   virtual SandboxFlags getSandboxFlags() const = 0;
   virtual void dispatchLoad() = 0;
 
   // On load failure, a frame can ask its owner to render fallback content
   // which replaces the frame contents.
+  virtual bool canRenderFallbackContent() const = 0;
   virtual void renderFallbackContent() = 0;
 
   virtual ScrollbarMode scrollingMode() const = 0;
   virtual int marginWidth() const = 0;
   virtual int marginHeight() const = 0;
   virtual bool allowFullscreen() const = 0;
   virtual AtomicString csp() const = 0;
   virtual const WebVector<WebPermissionType>& delegatedPermissions() const = 0;
 };
 
 class CORE_EXPORT DummyFrameOwner
     : public GarbageCollectedFinalized<DummyFrameOwner>,
       public FrameOwner {
   USING_GARBAGE_COLLECTED_MIXIN(DummyFrameOwner);
 
  public:
   static DummyFrameOwner* create() { return new DummyFrameOwner; }
 
   DEFINE_INLINE_VIRTUAL_TRACE() { FrameOwner::trace(visitor); }
 
   // FrameOwner overrides:
   void setContentFrame(Frame&) override {}
   void clearContentFrame() override {}
   SandboxFlags getSandboxFlags() const override { return SandboxNone; }
   void dispatchLoad() override {}
+  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 {
     DEFINE_STATIC_LOCAL(WebVector<WebPermissionType>, permissions, ());
     return permissions;
   }
 
  private:
   // Intentionally private to prevent redundant checks when the type is
   // already DummyFrameOwner.
   bool isLocal() const override { return false; }
   bool isRemote() const override { return false; }
 };
 
 }  // namespace blink
 
 #endif  // FrameOwner_h