Revert 253581 "Reorganize net/disk_cache into backend specific d..."

> Reorganize net/disk_cache into backend specific directories.
> 
> We want to unlink the blockfile cache on Android, and it seems that
> we'll want more platforms with more varieties of backend in the
> future. So let's move the backend into its own subdirectory as we
> start to untangle the dependencies from tests/etc... into their
> respective backends.
> 
> This initial checkin isn't perfect; in particular the tests have a lot
> of v2 specific dependencies still.
> 
> R=rvargas@chromium.org, cbentzel, rvargas
> BUG=331062
> 
> Review URL: https://codereview.chromium.org/121643003

TBR=gavinp@chromium.org

Review URL: https://codereview.chromium.org/181503005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253598 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253598