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 68d15341867d2ada8dbf8d081c790e7cd3eb8f52..3a97ae85bcc0ca6b09b0973bfce317d86b9b6fbd 100644
--- a/third_party/WebKit/public/web/WebLocalFrame.h
+++ b/third_party/WebKit/public/web/WebLocalFrame.h
@@ -10,6 +10,7 @@
 #include "WebFrameLoadType.h"
 #include "WebHistoryItem.h"
 #include "public/platform/WebCachePolicy.h"
+#include "public/platform/WebURLError.h"
 
 namespace blink {
 
@@ -122,6 +123,10 @@ class WebLocalFrame : public WebFrame {
                         WebHistoryLoadType = WebHistoryDifferentDocumentLoad,
                         bool isClientRedirect = false) = 0;
 
+  // On load failure, attempts to make frame's parent rendering fallback content
+  // and stop this frame loading.
+  virtual bool maybeRenderFallbackContent(const WebURLError&) const = 0;
+
   // Navigation State -------------------------------------------------------
 
   // Returns true if the current frame's load event has not completed.