PlzNavigate: Allow frames to fallback to alternative content.

content/renderer/render_frame_impl.cc

Index: content/renderer/render_frame_impl.cc
diff --git a/content/renderer/render_frame_impl.cc b/content/renderer/render_frame_impl.cc
index 06eba94a764b63205987137d32bc4b980f61c73a..138b3b0df2cb4fc3a4899f1e9db535bb3583a001 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -5063,6 +5063,14 @@ void RenderFrameImpl::OnFailedNavigation(
       common_params, std::unique_ptr<StreamOverrideParameters>(),
       frame_->isViewSourceModeEnabled());
 
+  // On load failure, a frame can ask its owner to render fallback content.
+  // If it is possible, then there is no more need to load an error page.
+  if (frame_->canRenderFallbackContent()) {
+    frame_->loadFailed();
+    browser_side_navigation_pending_ = false;
+    return;
+  }
+
   if (!ShouldDisplayErrorPageForFailedLoad(error_code, common_params.url)) {
     // The browser expects this frame to be loading an error page. Inform it
     // that the load stopped.