Lots of misc. history cleanup:
* Remove HistoryService::UnloadBackend(), and fold it into Cleanup(), which is
now its only caller since the memory purger is gone. This in turn means we
can nuke all instances of lazy-loading the backend.
* Use scoped_ptr<> more to indicate ownership transfers
* Use PostTaskAndReplyWithResult() to eliminate some cruddy wrapping code (there
is still more cruddy wrapping code).
BUG=350455
TEST=none
R=sky@chromium.org
Review URL: https://codereview.chromium.org/
195873002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256888 0039d316-1c4b-4281-b951-d872f2087c98