Add PolicyDictionaryMultipleSourceMergeList on crOS

Bug: 969891
Change-Id: Ic6a0d8b7428b7a474c8e3044ca8a8aa2d1de20b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1641990Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Yann Dago <ydago@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666090}

components/policy/resources/policy_templates.json

@@ -6266,7 +6266,7 @@
           'caption': '''Screen lock delays''',
         },
       ],
-      'supported_on': ['chrome.*:76-'],
+      'supported_on': ['chrome.*:76-', 'chrome_os:76-'],
       'features': {
         'dynamic_refresh': True,
         'per_profile': True,