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 f10589b80ab246a62641b8e0061c1129fe971ee0..66b6e0b8371dedeedbd428eeab602bed1e118372 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -5070,6 +5070,10 @@ void RenderFrameImpl::OnFailedNavigation(
   if (request_params.page_state.IsValid())
     history_entry = PageStateToHistoryEntry(request_params.page_state);
   LoadNavigationErrorPage(failed_request, error, replace, history_entry.get());
+
+  // On load failure, a frame may render fallback content which replaces the
+  // navigation error page.
+  frame_->didFailNavigation();

[clamy, 2016/10/21 16:09:42]

I'm a bit worried about how this interacts with the navigation history. In
particular do we create/update the FrameNavigationEntry in this particular case?
If we don't create or update any, could there be some url spoof issue?

[nasko, 2016/10/21 17:00:04]

Should we be loading an error page (last statement before this one) and then
replacing it with fallback content?

I think having a more explicit call to Blink, which returns a boolean whether
fallback content was rendered or not is a bit better. This will allow us to
display that or if no fallback, display the error page.

japhet@ and dcheng@ however are much more knowledgeable in this area, so I'd
defer to them.

[Nate Chapin, 2016/10/21 18:53:59]

That seems reasonable to me.

 }
 
 WebNavigationPolicy RenderFrameImpl::decidePolicyForNavigation(