Refactored DeviceManagementService to avoid basic_http_user_agent_settings.h.

The policy code will be moved into a component and can't include chrome/
code directly. This change moves the creation of BasicHttpUserAgentSettings into
the BrowserPolicyConnector, and makes the DeviceManagementService get the
abstract interface instead.

BUG=271392

Review URL: https://codereview.chromium.org/24592002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226108 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226108