Support policies referencing external data for device-local accounts

This CL adds a DeviceLocalAccountExternalDataService that provides each
device-local account with a DeviceLocalAccountExternalDataManager which
fetches, caches and retrieves external policy data.

BUG=256635
TEST=New browser test
R=derat@chromium.org, joaodasilva@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=233048

Review URL: https://codereview.chromium.org/25242002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233054 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233054