Archived History files removed from Chrome v37

Chrome v37 was released last week and the new version has many stability fixes and improvements, as well as 50 security fixes.  It also brings the end of the 'Archived History' database, meaning Chrome no longer keeps URL visit records older than three months.  As part of the upgrade to v37, Chrome will delete the existing 'Archived History' DB from disk when it starts.

This change will make it more difficult for investigators to recover older URL records.  However, 'Archived History' only kept URL records older than three months; the 'History' file (containing newer URL records) also has all the download records going back to when Chrome was first installed, so information on old downloads is still preserved.  Likewise, old cookies, Local Storage records, and bookmarks (which are stored in different locations) are still preserved.

Hindsight continues to work on Chrome v37; the removal of the 'Archived History' doesn't break anything.  However, a new release with some added features should be out in a couple of days.

The relevant revision (275159):

Eliminate the archived history database and clean up related code.

More specifically, this change list:
 * Removes all code related to the archived history database, and removes all mentions of it from comments.
 * Adds some code to purge the legacy database file on disk on first start-up, plus a unit test to check that this works.
 * Removes a migration path from M17 which was responsible for migrating some components, all of which have been unimplemented since then, except for the archived database, which, however, is now being removed.
 * Removes a tiny amount of code and some mentions of a "supplementary URL index" on the URLs table (this was unimplemented a long time ago). This is needed because otherwise some updated comments would be confusing.