Support for history deletions

[thomcc]

It's really unlikely that people will accept not being able to delete their history.

We probably need (at a minimum) deleteAll, deleteRange, and deleteUrl, a la webextension history apis.

[mhammond]

Alex has a document at https://docs.google.com/document/d/1b8jHJhE9Iszg4KLsg_KDsWshWA-BS49XBQ4ttJNPLL4/edit#, which is still a WIP and I've added some notes about the limitations of current sync in that document. I believe we are going to end up in the following scenario (which is, roughly, desktop parity):

• We will expose a way to delete all history. There's an open question about whether to sync this or not as the UX outcomes aren't clear - syncing this means all devices lose all history, and not syncing it (like desktop) means some of the history will slowly filter back in via other devices (eg, as those other devices re-visit the deleted page). IMO, a better UX here is to actually make "delete all history" to be a "clear history and disconnect from sync" UI, which sidesteps some of this - but regardless, there will be an API to delete all history.

• We will expose a way to delete a page (ie, without consideration to time ranges). Depending on agreed UX, we could take a param here to indicate whether the deletion should be synced or not. In that case, we'd probably add a new column to the tombstones table to indicate whether it should be synced or not.

• We will expose an API to delete visits in a time range. This implementation will delete visits, and also delete any pages which have zero remaining visits after the deletion, leaving behind a tombstone that can be synced. However, as we can't sync visit deletions for pages which remain, it doesn't make sense to ask whether to sync this deletion or not, because we will be unable to sync the deletion in all cases (ie, when visits for a page other than those deleted remain.)

[rfk]

From triage: this is done; @thomcc to link PR for completeness.