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 3d452f434c421353e633943028ebe7f1a38275e1..bd9cee20b52afe7ea2eba7fe6cdb3e668022dda5 100644
--- a/third_party/WebKit/public/web/WebLocalFrame.h
+++ b/third_party/WebKit/public/web/WebLocalFrame.h
@@ -123,6 +123,13 @@ class WebLocalFrame : public WebFrame {
                         WebHistoryLoadType = WebHistoryDifferentDocumentLoad,
                         bool isClientRedirect = false) = 0;
 
+  // Check if the frame's parent can render fallback content on load failure.
+  virtual bool canRenderFallbackContent() const = 0;
+
+  // Allow the embedders to report that a navigation they handle has failed. The
+  // frame's parent will display fallback content, if possible.
+  virtual void loadFailed() = 0;
+
   // Navigation State -------------------------------------------------------
 
   // Returns true if the current frame's load event has not completed.