Remove unnecesary usage of RefPtr and PassRefPtr of DatabaseContext.

This is a preparation to move DatabaseContext to Oilpan heap.

We don't need to use RefPtr/PassRefPtr of DatabaseContext for function arguments
and function return value. They don't move ownership of DatabaseContext. They
might be meaningful if we didn't have self->ref() in DatabaseContext::create.

Also, this CL simplifies DatabaseContext::backend, and makes
DatabaseManager::databaseContextFor private.

BUG=347902
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/209043008

git-svn-id: svn://svn.chromium.org/blink/trunk@170020 bbb929c8-8fbe-4397-9dbb-9b2b20218538