Move PolicyHeaderService/PolicyHeaderIOHelper to policy/core/common/cloud.

BUG=326799
TBR=jochen

Review URL: https://codereview.chromium.org/109993007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240612 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240612