Re: [w3c/manifest] Feature request: "isolate web app" hint (Issue #1109)

Putting syntax aside, it seems the main contention of Marcos' comment is: whether or not an explicit "don't isolate" option makes sense (distinct from a default value of "don't care"). Since this is just a hint, a value of "don't isolate" will not be contractually binding anyway, so we could argue that "don't isolate" and "don't care" will be roughly equivalent.

We can imagine four possible implementations:

1. Never isolates no matter what (ignoring this field). e.g. current Chrome.
2. Always isolates no matter what (ignoring this field). e.g. current macOS.
3. Doesn't isolate by default, but does isolate apps if they request it in the manifest. e.g. what we are tentatively planning for ChromeOS.
4. Isolates by default, but does not isolate apps if they opt out it in the manifest.

The reason why this field is useful (and not just isolating all apps) is because we know that isolation can break some apps if they depend on interaction with other origins. For now, we are planning to let sites opt in to this potentially breaking behaviour, letting them test to make sure everything still works before turning this on.

The question here is: do we ever foresee a use for 4? For now, no such implementation is planned. But I could imagine we'd get to a point where we want to turn on isolation by default, but give apps an opt-out in case they are broken. For that reason, I think there is value in making this a three-value enum. (Or, a two-value enum, with the ability to add a third "don't isolate" value later if we want to implement 4.)

On the syntax (which is in the territory of bikeshedding but we can get into it here): I think if we're going to only have "isolate" or "don't care" then I would probably stick with a Boolean with `false` as the default. The enum was an attempt to give us a 3-way choice of "isolate", "don't isolate" and "don't care". Let's figure out whether we want an explicit opt-out option first, then circle back to the syntax.

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