Remove disk_cache::BackendImpl::OpenPrevEntry() and all users.
While working on https://codereview.chromium.org/
542733002/ to fix
issue 410276, a question came up about how important OpenPrevEntry was
to maintain; it made that CL more complex and for what users?
It turns out, it's not really used. See
https://groups.google.com/a/chromium.org/forum/#!topic/net-dev/wlMR_zNJ85k
for a discussion.
Let's remove it and save ourselves some headaches.
R=clamy@chromium.org
BUG=410276
Review URL: https://codereview.chromium.org/
547513002
Cr-Commit-Position: refs/heads/master@{#293593}