Add PolicyListMultipleSourceMergeList support for crOS

Bug: 969889
Change-Id: I0df8a98a007691a2153b67caeaea3e22ea7ccadd
TBR: pastarmovj@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1641988
Commit-Queue: Yann Dago <ydago@chromium.org>
Reviewed-by: Yann Dago <ydago@chromium.org>
Reviewed-by: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665672}

components/policy/resources/policy_templates.json

@@ -6150,7 +6150,7 @@
         'type': 'array',
         'items': { 'type': 'string' },
       },
-      'supported_on': ['chrome.*:75-'],
+      'supported_on': ['chrome.*:75-', 'chrome_os:75-'],
       'features': {
         'dynamic_refresh': True,
         'per_profile': True,