External Print Servers: Bugfixes in the new policy

Two bugfixes to the new policy ExternalPrintServers:
- Unnecessary definition of kExternalPrintServers was removed from
  pref_names.*.
- The type of the policy was set to 'external'; the previous value
  'dict' was incorrect.
Also activating this policy in M78 (by removing 'future' field in
policy_templates.json) and updating an entry for this policy in the
file policy_test_cases.json.

BUG=chromium:902085
TEST=on local chrome build

Change-Id: I087318282f43b46ea3772295aba223519f4a6065
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1779723
Commit-Queue: Sergey Poromov <poromov@chromium.org>
Reviewed-by: Sergey Poromov <poromov@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Auto-Submit: Piotr Pawliczek <pawliczek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692878}
4 files changed