Move policy code into components/policy.

The code moved to component/policy can be included on iOS, but can't depend
on chrome/. This move includes an exception for the policy protobufs:

- the generic cloud policy protobufs will be moved into a new repository,
  due to a dependency from a ChromeOS package

- the user cloud policy protobuf and its decoding will move into a new component
  for user policy

BUG=271392
TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/109743002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239399 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239399