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 1115f80527d5811b2a518473947ab65682847fd5..f30de886c4784faeab1a8944be992e21a91b411d 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -5049,6 +5049,13 @@ void RenderFrameImpl::OnFailedNavigation(
     return;
   }
 
+  // On load failure, a frame can ask its owner to render fallback content.
+  // When that happens, don't load an error page.
+  if (frame_->maybeRenderFallbackContent(error)) {
+    browser_side_navigation_pending_ = false;
+    return;
+  }
+
   // Make sure errors are not shown in view source mode.
   frame_->enableViewSourceMode(false);