Auto merge of #11544 - jdm:privatebrowsing, r=asajeffrey

Implement private browsing for mozbrowser

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Support the `mozprivatebrowsing` attribute on mozbrowser iframes. This separates the non-private and private sessions in terms of cookies, HSTS lists, cached HTTP credentials, HTTP connection pools, and web storage. The private session is shared between all private mozbrowsers, and lasts until shutdown.

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