Re: [w3ctag/design-reviews] hasDroppedEntry in PerformanceObserverCallback (#547)

@kenchris, @torgo, @rhiaro and I discussed this in our VF2F breakout today. Here is our feedback:

- It may be good to prefer a dictionary over adding a positional primitive argument, in line with [Principle 5.3](https://w3ctag.github.io/design-principles/#prefer-dict-to-bool). While some of the reasoning behind this principle doesn't apply here (since this callback is only called by the UA), a dictionary would allow communicating more information to the callback in the future, without introducing a long list of positional arguments that need to be backfilled.
- We wondered if it would be more in line with your listed goals to report the *number* of dropped entries, so appropriate optimizations can be made. E.g. the need for buffer adjustments is higher when there are thousands of dropped entries compared to a couple. We also wondered if it would assist in reducing the number of dropped entries if developers could set the buffer size via an `observe()` parameter.
- A little of the discussion revolved around naming. If this changes to communicate number of dropped entries, it could change to e.g. `droppedEntries`. The word `truncated` could also be useful in coming up with alternative composite names.

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