Mark DeviceBlockDevmode support by google_cloud only

Active Directory devices apply the setting that come from device
cloud policy.

BUG=chromium:889893

Change-Id: I0738f35dfdad56042c388699f811b43f3a530501
Reviewed-on: https://chromium-review.googlesource.com/c/1365595
Commit-Queue: Roman Sorokin <rsorokin@chromium.org>
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614681}

components/policy/resources/policy_templates.json

@@ -7061,6 +7061,7 @@
       'type': 'main',
       'schema': { 'type': 'boolean' },
       'supported_on': ['chrome_os:37-'],
+      'supported_chrome_os_management': ['google_cloud'],
       'device_only': True,
       'features': {
         'dynamic_refresh': True,