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..de1d292ad83319b26462ab7ea8ea82e97261cc8a 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -5071,6 +5071,15 @@ void RenderFrameImpl::OnFailedNavigation(
     return;
   }
 
+  // On load failure, a frame can ask its owner to render fallback content.
+  // If it is possible, there is no more need to load an error page since it
+  // will not be displayed.
+  if (frame_->canRenderFallbackContent()) {
+    frame_->loadFailed(error);
+    browser_side_navigation_pending_ = false;
+    return;
+  }
+
   // Make sure errors are not shown in view source mode.
   frame_->enableViewSourceMode(false);