Also forgot to mention, this happens quite consistently with maps page in v8.runtimestats.top_25 benchmark on the cold run. The attached trace is from one such run.

It seems the trace event 'navigationStart' with category 'user_timing' is missing from the trace. I believe this is emitted here: https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/loader/DocumentLoadTiming.cpp?type=cs&q=navigationStart+blink.user_timing&sq=package:chromium&l=120

I can reproduce the problem on Tip of Tree chrome: 
1. Restart chrome (do not use an already running instance)
2. navigate to about:tracing. Select all the default categories. Start recording.
3. Open a new tab. Type in "google.com". Press Enter.
4. Wait a few seconds, then go back to about:tracing. Stop recording.
5. Open the metrics panel on the right. See everything is empty. You can also verify that no 'navigationStart' event is in the trace. 

What's surprising is if you now start recording again, go back to the google tab, and refresh it, and then stop recording, the navigation start event is now there and you see the metrics in the metrics panel.

Somehow the trace event is not emitted in the first case, and I didn't get a chance to debug yet why. 

Deep, are you a good owner for this?

I will look into this more and assign to a more appropriate owner if needed.