Simple Cache: Turn some DCHECKs into CHECKs

We're trying to chase down http://crbug.com/317138.  Make some relevant
DCHECKs into CHECKs in hopes of an earlier crash.

BUG=317138

Review URL: https://codereview.chromium.org/70183004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235808 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235808