Bug 1536468 - Make retrieval of WebBrowserPersistDocumentAttrs.cacheKey async in SessionHistory, r=peterv

Differential Revision: https://phabricator.services.mozilla.com/D24472

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extra : rebase_source : bf5ac32a5e89ec16c07d55d631e40014f729e75f
extra : source : b1bb781543ed7c2ad8dcc3e3146d7e3f4973195d