Background context:go/memory-infra: memory profiling in chrome://tracing

I was trying to trace my daily actions with heap profiling enabled, taking a trace every 30 mins.
I found that memory increased between 4-6am in my Chrome when I was not using. But it is mainly due to the news sites updating.

What is worrying is that the private dirty shows an increase and none of the categories account for the increase.

Trace file:
https://drive.google.com/open?id=0B7f4beGia2iHS0JsTHV1MWNJQmc

Check the memory usage of renderer pid: 31596 page: new york times. Check the difference between the last two trace points and see the increase in private dirty memory can't be explained.

Could it be due to the shared memory that we are not accounting? Can it count as private dirty? It is directly mmaped, not malloced. I am not sure what else mmaps memory.