PlzNavigate: Allow frames to fallback to alternative content.

third_party/WebKit/Source/web/WebLocalFrameImpl.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 245 matching lines @@
   void loadData(const WebData&,
                 const WebString& mimeType,
                 const WebString& textEncoding,
                 const WebURL& baseURL,
                 const WebURL& unreachableURL,
                 bool replace,
                 WebFrameLoadType,
                 const WebHistoryItem&,
                 WebHistoryLoadType,
                 bool isClientRedirect) override;
+  bool maybeRenderFallbackContent(const WebURLError&) const override;
   bool isLoading() const override;
   bool isFrameDetachedForSpecialOneOffStopTheCrashingHackBug561873()
       const override;
   bool isNavigationScheduledWithin(double interval) const override;
   void setCommittedFirstRealLoad() override;
   void setHasReceivedUserGesture() override;
   void sendOrientationChangeEvent() override;
   WebSandboxFlags effectiveSandboxFlags() const override;
   void forceSandboxFlags(WebSandboxFlags) override;
   void requestRunTask(WebSuspendableTask*) const override;
@@ skipping 199 matching lines @@
 
 DEFINE_TYPE_CASTS(WebLocalFrameImpl,
                   WebFrame,
                   frame,
                   frame->isWebLocalFrame(),
                   frame.isWebLocalFrame());
 
 }  // namespace blink
 
 #endif