Move policy strings to components/policy_strings.grdp.

This facilitates the refactoring of chrome/browser/policy into a layered
component.

BUG=271392
TBR=joi

Review URL: https://codereview.chromium.org/85753005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237336 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237336