External Print Servers: Activate related policy for R-79

This CL activates ExternalPrintServers policy.

BUG=chromium:902085
TEST=none

Change-Id: I0f273326e8984f3bf6c5d99a7c1fd513e73216f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1848006
Auto-Submit: Piotr Pawliczek <pawliczek@chromium.org>
Commit-Queue: Sergey Poromov <poromov@chromium.org>
Reviewed-by: Sergey Poromov <poromov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704120}

chrome/test/data/policy/policy_test_cases.json

@@ -428,7 +428,13 @@
   },
 
   "ExternalPrintServers": {
-    "note": "This policy will be added in R-79"
+    "os": ["chromeos"],
+    "test_policy": {
+      "ExternalPrintServers": {
+        "url": "https://example.com/policyfile",
+        "hash": "deadbeefdeadbeefdeadbeef"
+      }
+    }
   },
 
   "NativePrinters": {

components/policy/resources/policy_templates.json

@@ -17747,7 +17747,6 @@
       },
       'id': 572,
       'supported_on': ['chrome_os:79-'],
-      'future': True,
       'features': {
         'dynamic_refresh': True,
         'per_profile': True,