PlzNavigate: Allow frames to fallback to alternative content.

third_party/WebKit/public/web/WebLocalFrame.h

Index: third_party/WebKit/public/web/WebLocalFrame.h
diff --git a/third_party/WebKit/public/web/WebLocalFrame.h b/third_party/WebKit/public/web/WebLocalFrame.h
index 1e6d90dc3233296bfb90a4e658a30a14572721e7..ef994fb7789047051f48b79602f031002abc63f4 100644
--- a/third_party/WebKit/public/web/WebLocalFrame.h
+++ b/third_party/WebKit/public/web/WebLocalFrame.h
@@ -123,6 +123,10 @@ class WebLocalFrame : public WebFrame {
                         WebHistoryLoadType = WebHistoryDifferentDocumentLoad,
                         bool isClientRedirect = false) = 0;
 
+  // Allow the embedders to report that a navigation they handle has failed. It
+  // permits the frame to display fallback content instead of an error frame.
+  virtual void didFailNavigation() = 0;
+
   // Navigation State -------------------------------------------------------
 
   // Returns true if the current frame's load event has not completed.