Long running unload handler can cause incognito BrowserContext to be reused

We discard the incognito BrowserContext when there are no more WebViews using it, but we have to delay discarding it until all render processes using it have gone away.

It's possible for a long running unload handler to keep the BrowserContext alive for long enough for it to be re-used in situations when the application should expect to get a fresh incognito BrowserContext (ie, opening an incognito WebView after closing all existing ones).