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..55d522013aaf9d479329306f5d621ae39a92f2a4 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -5071,6 +5071,14 @@ 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

[clamy, 2016/11/04 14:47:12]

Nit: Rewrite the second sentence as "When that happens, don't load an error
page..."

[arthursonzogni, 2016/11/07 09:59:06]

Done.

+  // will not be displayed.
+  if (frame_->maybeRenderFallbackContent(error)) {
+    browser_side_navigation_pending_ = false;

[clamy, 2016/11/04 14:47:11]

Will maybeRenderFallbackContent eventually lead to a DidCommitProvisionalLoad or
DidStopLoading IPC be sent? If not, we should send a FrameHostMsg_DidStopLoading
here, since the browser expects us to be loading something and needs to be told
that the load has stopped.

[arthursonzogni, 2016/11/07 09:59:06]

Yes it is. The list of calls is:
 * WebLocalFrameImpl::maybeRenderFallbackContent
 * FrameLoader::loadFailed
 * ProgressTracker::progressCompleted
 * FrameLoaderClientImpl::didstopLoading
 * RenderFrameImpl::didStopLoading

It leads to send a DidStopLoading IPC

+    return;
+  }
+
   // Make sure errors are not shown in view source mode.
   frame_->enableViewSourceMode(false);