Changed CloudPolicyValidator to use SHA256 for verification signatures.

BUG=275291

Review URL: https://codereview.chromium.org/132803009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249672 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249672