Reland re-organization of net/disk_cache/

Originally this landed from https://codereview.chromium.org/121643003/ ,
but reverted due to an iOS build break.

BUG=331062
TBR=mmenke@chromium.org,rvargas@chromium.org

Review URL: https://codereview.chromium.org/185003007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254474 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254474