Disk cache: Downgrade a DCHECK to a DLOG.

Once in a while this hits a developer after going through a system crash (or
forced restart), as expected, but the rest of the code solves the issue
without user intervention.

BUG=none
TEST=none
R=jar@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=237274

Review URL: https://codereview.chromium.org/85643005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237430 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237430