Extract AutofillPolicyHandler into its own file.

Note that the tests have been refactored to avoid reliance on the
ConfigurationPolicyPrefStore.

BUG=271392

Review URL: https://codereview.chromium.org/40423002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230973 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230973