Add CloudExternalDataPolicyObserver

This CL adds the CloudExternalDataPolicyObserver. This helper can be used
to observe a policy referencing external data for all users on a device,
covering both device-local accounts (whose policy is available and may
change anytime) and regular accounts (whose policy is available and may
change only when the user is logged in).

The helper emits notifications when an external data reference is set,
cleared or an external data fetch completes successfully.

The helper will be used to implement a policy for setting the avatar image
and a policy for setting the wallpaper.

The CL also adds the first policy referencing external data. The policy is
not wired up yet but having it defined unblocks server-side work and makes
testing easier. The policy will be implemented in a follow-up CL coming
soon.

BUG=152957,152959,220418
TEST=Full unit test coverage
TBR=askvitkine@chromium.org

Review URL: https://codereview.chromium.org/88423002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237806 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237806