Would this be a good first bug?

Yes, it's perfect :)

Over to Max, we can discuss in our next 1-1.

Thanks! I will take a look.

Logic belongs in here:
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/timing/PerformanceBase.cpp?rcl=00555778f4fc32501162d851633180b1fe0cec9f&l=436

What's the current status of this change? Is it blocked on anything? I know there is some spec work happening around this, but I don't think it should block implementation?

Yes, the spec is not finalized yet, as being discussed in this thread - https://github.com/w3c/performance-timeline/issues/81.

As the feature prototyped behind a flag, should we just implement the not-yet finalized proposal or wait till it finalize?

I think implementing the current proposal behind the flag would make sense.