Re: [w3c/ServiceWorker] New "setup" lifecycle for service worker (#1576)

My initial problem is that if multiple events come **at the same time**, `await readDataFromStorage()` will be called multiple times if I don't use my initial snippet in this post.

Your solution, in that condition, the `init_promise` will be awaited multiple times. `await` a resolved promise is Ok, although not common.

Use your method, my code will look like:
```
var cache;
var init_promise = async function() {
  cache = await readDataFromStorage();
}();

chrome.tabs.onUpdated.addListener(function(parameters) {
  await init_promise();
  // use cache data process events
});
```
Your solution is more simple than my initial solution. But if service worker support 'setup' phase, no need to use an init_promise, and everything's ready, that is more easy to understand.

I think 'setup phase' like 'top level await' in service worker, they do the same thing. For example, the code can like below
```
var cache = await readDataFromStorage();
// then listen events
```
So 'setup phase' vs 'top level await', which is better?


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