[iOS CBCM] Add support for CloudPolicyOverridesPlatformPolicy

Adds support for the CloudPolicyOverridesPlatformPolicy policy, which
controls whether policies fetched from CBCM should take precedence over
policies found on the device.

This policy is a requirement for launching CBCM. For now, it is
declared as a Future policy for iOS until iOS CBCM is launched.

Bug: 1066495
Change-Id: I5b2ce20e2f2b435992f84747a47a3c2ea0f14273
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2332322Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Cr-Commit-Position: refs/heads/master@{#794233}

components/policy/resources/policy_templates.json

@@ -16106,6 +16106,7 @@
       'type': 'main',
       'schema': { 'type': 'boolean'},
       'supported_on': ['chrome.*:75-'],
+      'future_on': ['ios'],
       'features': {
         'dynamic_refresh': False,
         'per_profile': False,

ios/chrome/test/data/policy/policy_test_cases.json

@@ -68,6 +68,8 @@
     ]
   },
 
+  "CloudPolicyOverridesPlatformPolicy": {},
+
   "DefaultPopupsSetting": {
     "os": [ "ios" ],
     "note": "TODO(crbug.com/106682): Flag this with can_be_recommended",