real fix for b/273369126 . (also resolves b/273583026)

[copybara-service[bot]]

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real fix for b/273369126 . (also resolves b/273583026)

removes the YieldUnsafeUnsortedEvents() API, removes MaybeDropEventsForTraceViewer, and distribute this logic between TraceContainer::EventSlice and the JSON serializer.

TraceContainer is now a safe ARC type, so we can build copy-on-write abstractions over it freely now. i did not end up giving EventSlice an iterator API, because it seemed more complex than what it is actually being used for at the moment, which is just JSON serialization. but in the future we could explore this direction.