Move DiskCacheDirPolicyHandler to live in chrome/browser/net.

NOTE: the unit tests are brand-new.

BUG=271392

Review URL: https://codereview.chromium.org/52253002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232089 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232089