Note that render_frame_impl.cc does not appear to have any code search annotations at all (no links at all when viewing it in code search).

Sigh, thank you for the report.

I thought we had alerting in place to detect when this happened?

Thanks for the report and sorry for this drop to of Xrefs. I'm suspecting a bad push of an index pack. 

We do have alerts and one for Xref alerts fired yesterday. We don't have an alert for Xrefs disappearing because it is tricky to setup based on the data we have.  

While investigating the Xrefs latency issue yesterday I may have found an issue related to this behavior, having to do with how index packs are pushed and picked up by the Xrefs pipeline.

It doesn't seem like the index pack push issue is the cause of this problem. The cause here is likely a compilation issue in the gen bot for chromium/Linux, e.g., 

"ninja: Entering directory `/b/build/slave/codesearch-gen-chromium-linux/build/src/out/Debug'
ninja: error: toolchain.ninja:65: loading 'obj/base/base_i18n_perftests.ninja': No such file or directory
subninja obj/base/base_i18n_perftests.ninja
                                           ^ near here"

Similar issues for revision 505268 - 505463, then the compile became successful again. Unfortunately, this can happen but we can work on improving the alerting and how to recover on failure.