Revert "Bug 930269 - Rename SessionStore.restoreHistoryPrecursor"

This reverts commit 3f7ebff8dbe7875e9e782928bb52544ef357b871.
Merge branch 'cleanup' into nightly
Bug 867097 - Remove superfluous __SS_tabStillLoading property
Bug 867118 - Remove browser.__SS_data and use a WeakMap instead
setTabValue/deleteTabValue