Resetting the global attribution API flag

[bmcase]

The spec has a TODO on defining how to reset the user activation flag.

From @martinthomson comment on this topic:

I think that we have reset conditions that come for free:

Navigation causes the value to clear, because that creates a new Window object.
That is probably OK for now. What we might consider for reset include:

Extra long periods without interaction, such that the site execution is paused.
What we might want to avoid:

Browser restarts. If those reactivate sessions, the site will expect the API to function.